GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 12, NO. 3, PAGES 479-499, SEPTEMBER 1998
Meridional transport of dissolvedinorganic carbon
in the South Atlantic
Ocean
J. Holfort,1'2K. M. Johnson,
1B. Schneider,
• G. Siedler,
4andD. W. R. Wallace1,2
Abstract. The meridionaloceanictransports
of dissolved
inorganiccarbonandoxygen
were calculatedusingsix transoceanic
sectionsoccupiedin the SouthAtlanticbetween
11øSand30øS. The totaldissolvedinorganiccarbon(TCO2)datawere interpolatedonto
conductivity-temperature-depth
datato obtaina high-resolutiondata set, and Ekman,
depth-dependent
anddepth-independent
components
of the transportwere estimated.
Uncertainties
in the depth-independent
velocitydistribution
werereducedusingan
inversemodel. The inorganiccarbontransportbetween11øS and30øS was southward,
decreased
slightlytowardthesouth,andwas-2150 _+200 kmols4 (-0.81 _+0.08 Gt C
yr4) at 20øS.Thisestimate
includes
thecontribution
of netmasstransport
requiredto
balancethe salttransportthroughBeringStrait. Anthropogenic
CO2concentrations
were
estimatedfor the sections.The meridionaltransportof anthropogenic
CO2wasnorth-
ward,increased
towardthenorth,andwas 430 kmols-• (0.16 Gt C yr4) at 20øS.The
calculations
implynetsouthward
inorganic
carbontransport
of 2580kmols4 (1 Gt C yr-•)
duringpreindustrialtimes.The slightcontemporary
convergence
of inorganiccarbon
between10øSand 30øSis balancedby storageof anthropogenic
CO2and a sea-to-airflux
implyinglittle local divergenceof the organiccarbontransport.During the preindustrial
era, therewas significantregionalconvergence
of bothinorganiccarbonandoxygen,
consistent
with a sea-to-airgasflux drivenby warming.The northwardtransportof
anthropogenic
CO2carriedby the meridionaloverturningcirculationrepresentsan
importantsourcefor anthropogenic
CO2currentlybeingstoredwithin the North Atlantic
Ocean.
1.
however,to be usefulin analyzingthe globalcarboncycle,
suchan analysisrequiresan assessment
of thecorresponding
Introduction
As high-qualitydataon the globaldistribution
of CO2 in
the atmospherebecameavailable, it becamepossibleto
combinethesedatawithatmospheric
transportmodelsto infer
the transportof CO2 from onepart of the globeto another
[e.g., Keelinget al., 1989; Tanset al., 1990;Entinget al.,
1995]. This approachpromisesto provideinsightinto the
regionaldistribution
of sources
andsinksof atmospheric
CO2;
transportof CO2 by the oceancirculation[Broeckerand
Peng, 1992].
The oceanic transportof chemicalpropertiescan be
approached
usingmethodssimilarto thosedeveloped
for the
calculationof heatandwatertransports
[e.g., Wunsch,1978;
Hall andBryden,1982;Roemmich
and Wunsch,1985].This
approachhas been appliedto the transportof nutrients
[Rintouland Wunsch,1991; Wunschet al., 1983; Robbins
carbon[Breweret al., 1989;
•Department
of AppliedScience,Brookhaven
NationalLaboratory, andBryden,1994]andinorganic
Martel and Wunsch, 1993; Robbins, 1994; Holfort et al.,
1994; $toll et al., 1996]. The latter made use of measurements of the total dissolvedinorganic carbon content of
3Sektion Meereschemie,Institut fOr Ostseeforschung,Rostock- seawater(TCO2).
Warnemtlnde,Germany.
Someof theseanalysesof inorganiccarbontransports
have
4InstitutfOr Meereskundean der UniversitatKiel, Kiel Germany.
been limited by a lack of high-quality,denselysampled
Upton,New York.
2 Now at Institut fOr Meereskunde an der Universitat Kiel,
Germany.
Kiel,
sections
with TCO2 measurements,
aswell asby incomplete
specification
of thevariouscomponents
of thetransport
which
can contributeto the meridionaltransportof CO2. Densely
sampled,high-qualityTCO2 dataare, however,now being
collected in conjunctionwith World Ocean Circulation
Experiment(WOCE) cruises,by theJointGlobalOceanFlux
Study(JGOFS)globalsurveyof CO2in theoceans[Sabineet
Copyright1998 by theAmericanGeophysical
Union.
Papernumber98GB01533.
0886-6236/98/98GB-01533 $12.00
479
480
HOLFORT ET AL.: SOUTH ATLANTIC CO2 TRANSPORT
al., 1997]. An exampleof the use of transportestimates operatormultiparameter
metabolicanalyzersystems
[Johnson
calculated
usingmodernsection
datawasrecentlyreported
by and Wallace, 1992; Johnsonet a/.,1993]. On the WOCE
$toll et al. [1996].
cruises,theTCO2 analyses
werecross-checked
for accuracy
In thispaper, we use a setof thesenew WOCE sections, againstCertified ReferenceMaterials provided by A.
withcorresponding
TCO2data,asthebasisforcalculating
the Dickson of Scripps Institutionof Oceanography.These
north-south
transport
of CO2withintheSouthAtlanticOcean analysessuggested
thatthe WOCE dataare accurateto better
(we alsoreportthetransportof dissolved
oxygen).We focus than 2 3tmolkg-•. Additionalsections
were takenfrom
a greatdealof anentionontheuncertainties
whichinevitably historical
data,particularly
thehigh-quality
pre-WOCETCO2
arisein suchcalculations.
The calculations
we presentare
datacollectedduringthe SouthAtlanticVentilationExperiappropriatefor the early 1990swhen the observations
were
ment (SAVE) (T. Takahashiand D. Chipman,personal
made.However,the concentration
of CO2 in the oceansis
communication,1991). We have comparedthe data from
increasingwith time as a resultof the uptakeof excessor
SAVE Legs2 and3 in theWesternBasinwith thetheWOCE
anthropogenic
CO2fromtheatmosphere,
andanthropogenic TCO2 data from the sameregionusinga multivariateapCO2 concentrations
and oceancurrentsare both spafially proachand foundno significantoffsetsbetweenthe datasets
variable. We therefore also estimated the concentration
[Wallaceet al., 1996]. Table 1 presentsa summaryof the
distribution
of anthropogenic
CO2 in the sections
andcalcu- sectionsused,and Figure 1 showstheir geographical
localatedits transport.On the assumption
thattheoceancircula- tions.
tion andthe ocean'snaturalcarboncyclehavenot changed
In orderto calculatematerialtransports
fromhydrographic
significantlysincepreindustrialtimes (e.g., since 1750), sectiondata,highdatadensities
aregenerallyrequired.Water
subtraction
of theanthropogenic
CO2transportfromthetotal chemistrydataare availablefor a maximumof ~36 discrete
transportmeasuredin the early 1990s providesan estimate depthsper station(> 100 m resolution),comparedto the ~2
of the inorganiccarbontransportwhich occurredduring dbarresolutionof the conductivity-temperature-depth
(CTD)
preindustrialtimes. In a separatepaper,we will discuss
the profiler data from which geostrophicwater transportsare
implicationsof the anthropogenic
CO2 transportfor the estimated.Althoughnutrients,oxygen,andtheCTD dataare
storageof anthropogenic
CO2 in theNorthAtlanticOcean.
availablefor almostall of thesebottlesamples,TCO2 measurements
are notavailablefor every stationandeverybottle
depth
because
of analysistime limitations.To increasethe
2. Data and Data Interpolation
TCO2 data densityprior to making the inorganiccarbon
The mainhydrographic
sections
usedfor thecalculation
of
transportcalculations,
someformof interpolation
or mapping
the meridionaltransportof inorganiccarbon in the South schemeis required. Recently, Goyet et al. [1995] demonAtlanticweretheWOCE sections
A8 (11øS),A9 (- 19øS)and
stratedthepotentialof quadraticspatialinterpolation
of sparse
A10 (~30øS). Thesesections
wereoccupiedby theInstitutf•r
TCO2 data. As they noted,however,consideration
of nearprofilesof "mastervariables"suchas T, S, and
Meereskunde,Kiel, Germany, on boardthe researchvessel continuous
Meteor between 1991 and 1994 [Siedler and Zenk, 1992;
02 shouldallowsignificant
improvement
overspatialinterpoSiedler et al. , 1993; Zenk and Mi•ller, 1995; Siedler et al. ,
lationschemes.Previousstudieswhichusedmultiplelinear
withsuchvariablesto interpolate
TCO2 ontobottle
1996]. The totaldissolved
inorganiccarbon(TCO2) values regression
[Johnsonet al., 1995, 1998] were determinedusingsingle- data were performedon basinscales[Breweret al., 1995;
ß
5øS
10øS
20øS
30øS
35øS
55øW50øW
40 øW
30øW
20øW
10øW
OøE
10øE
20øE
Figure 1. Map of the SouthAtlanticOceanshowing
thegeographical
positions
of thegeostrophic
velocities
usedin the
transportcalculations.
Each star represents
a locationmidwaybetweentwo adjacentstations.Pleasenote that there are
overlapping
sections
at 11øSandtowardtheeastern
endof 30øS(eastof 5øW).Themeridional
sections
areusedonlywiththe
conservation
constraints
in the inversemodel(seeappendix,constraintB).
HOLFORT
ETAL.'SOUTH
ATLANTIC
CO2TRANSPORT
481
Table 1. SectionData Usedfor thisStudy
Cruise
WOCE
Location
(s)
Date
Section
Bottles
TCO2
Tripped
Samples
Institutions
Meteor
28
A8
April1994
11øS
3840
1540
IfM / BNL
Meteor
15
A9
February
i99i
i9øS
3290
690
hqM/ BNL
Meteor
22
A10
January
1993
30øS
3420
1420
IfM / BNL
Oceanus
133
-
March1983
11øS,
23øS
3690
SAVE
Leg3
-
February
1988
25øS,
0øW
2560
0
1620
WHOI
SIO/LDEO
SAVE
Leg4
December
1988
30øS
2290
1040
SIO/LDEO
"Bottles
Tripped"
refers
toallwater
samples
collected
during
thecruise,
ofwhich
only
asubset
were
analyzed
forTCO
2.Note
that
theOceanus
sections
werenotsampled
forTCO2:
values
wereinterpolated
using
themethod
described
inthetextonthebasis
of
adjacent
section
data.
These
sections
were
used
because
thetransport
estimates
are
highly
contingent
onthemass
transport
fields:
hence
weused
allavailable
sections.
Abbreviations
areasfollows:
IfM, Institute
furMeereskunde,
Kiel;BNL,Brookhaven
National
Laboratory;
WHOI,
Woods
Hole
Oceanographic
Institution;
SIO,Scripps
Institute
ofOceanography;
andLDEO,
Lamont-Doherty
EarthObservatory.
Dashindicates
non-WOCE
cruises.
Wallace,1995].Theresiduals
forsuchlarge-scale
regressions rationalefor the choiceof thesepredictorvariablesis given
vary systematically
with geographic
position
and depth by Breweret al. [1995] and Wallace[1995]. Figure2
presents
a composite
plotof theresiduals
for theTCO2 data
[Wallace, 1995].
for oneof thezonalsections
We therefore
developed
a three-step
procedure
to mapthe derivedfrom theseregressions
sparse
TCO2 bottledataontotheCTD datausingmultiple usedin this study.The standarddeviationof the residuals
linearregression.
Ourapproach
involved
thedevelopment
of fromthesefitsfor all TCO2 datais 3 •mol kg-1,witha
for largerresiduals
withintheupper200m (Figure
localizedmultiparameter
linearregressions
from all the tendency
2) [cf. Breweret al., 1995].Thenow"complete"
bottledata
thenutrients
domaincenteredaroundthe position(latitude,longitude, setfor TCO2 wasusedin step2 to interpolate
For every
pressure,
ordepth)
atwhich
avalue
wastobecalculated.
A andTCO2 fieldsontothe CTD measurements.
CTD
depth
(2
dbar
resolution),
a
regression
with
independent
horizontaldomainextentof 2øx 2ø waschosento providean
adequate
number
of samples
foreachdomain
in mostcases. variablesAOU, $, !9, andao wasperformedusingthebottle
availablebottle data which were locatedwithin a specified
Thevertical
extent
oftheregression
domains
(2 x APreg
) was data that were located within the local domain (as defined
variedwith pressure,
starting
at + 150 dbararoundthe above)centeredon theparticularCTD depth.Finally,the
equations
wereappliedto theCTD profile
desired
position
close
totheseasurface
andincreasing
with localregression
depthaccordingto
APreg
= 150+ 0.1*P
(1)
whereP is the pressure.A variableverticalextentwas
datato estimate
a TCO2 concentration
for eachdepthin the
CTD record.Usingthisthree-step
method,we attainedan
effective
TCO2 datadensity
of -•50kmhorizontal
resolution
alongthe cruisetracksand2-10 dbarin the vertical(the
relativelyslowresponse
of theoxygensensor
likelyreduces
the effective vertical resolutionof the CTD-O 2 measure-
required
because
of thestronger
vertical
gradients
foundin ments).A similarapproachhasbeenrecentlyproposed
by
theupperocean
andthelowerresolution
ofthebottle
dataat
GoyetandDavis [ 1997];theprincipaldifferenceis ouruseof
greater
depths.
In cases
wheretherewereinsufficient
bottle
geographically
localizedregressions
throughout
the water
datatoconstrain
a regression
fit fora domain
of thissize,the
size of the domainwas doubled.This was necessaryfor the
Oceanuscruises,on which no TCO2 measurements
were
column.
madeandfor whichthe mappingontothe bottleandCTD
datareliedon regression
equations
derivedfromdatacol-
3.
lectedfromadjacent
cruise
tracks.A 5ø x 5ø areawasused
The methodfor calculatingthe CO2 transportwassimilar
to thatusedpreviouslyfor the calculation
of heatandsalt
transports
andwasbasedon techniques
introduced
by Bryan
[1962], Wunsch[1978],Roemmich
[1980],etc.for theNorth
Atlantic.Closelyrelatedcalculations
usingSouthAtlanticdata
have been performedby Fu [1981], MacDonald[1993],
Holfort [1994], Saunders
andKing [1995]andJ. Holfortet
al. (The heattransportin the SouthAtlanticOcean,manuscriptin preparation,
1998;hereinafter
referredto Holfortet
for theMeteor15 (19øS)cruise for whichthedatadensity
was relatively sparse.
In step1, we first usedmultiplelinearregression
to
calculate
thefew missing
nutrientdatawithinthebottledata
sets.We subsequently
estimated
TCO2 valuesfor all of the
bottlesamples
for whichtherewasnoTCO2 analysis.
The
latter was achievedusingregressions
with the following
independent
parameters:
apparent
oxygen
utilization
(AOU),
NO3,SiO4,salinity
(S),andpotential
temperature
(O).A
Method
a!., manuscritxin preparation1998). In particular,Holfort
482
HOLFORT ET AL.' SOUTH ATLANTIC CO2TRANSPORT
[1994] and Holfort et al. (manuscriptin preparation,1998)
presentedcalculations
of heat and freshwatertransports
for
the same sectionsdiscussed
here, togetherwith a more
transportacrossthe section.The compensating
velocitywas
defined as
Af0
deraileddiscussionof the calculationprocedures.It is worth
noting that the heat transportscalculatedfrom the sections
discussed
here agreevery well with thoseestimatedby other
workersfor similarlatituderanges.Thesehavebeensummarized and placed in a global and historical context by
- EK
Am -H
¾'comp
= AfO
MacDonald and Wunsch [1996].
Am -H
We usetheterminorganiccarbontransport(Tc) for thenet
transportof TCO2acrossan oceansection,thatis,
rc=f f v.
Am -H
rco2aaz
(5)
(2)
The Tc was calculatedby integratingthe productof total
Thedepth-independent
or "barotropic"
velocity•N• was
specifiedon the basisof an assumed level of no motion
(LNM). For the simplest LNM choice considered, the
"baroclinic"
case,thisbarotropiccomponent
wasspecified
to
be zero for each stationpair (see (4)). However, we employed34 additionalapproaches
to specifyinga level of no
motion[Holfort,1994].Briefly,theseapproaches
included(1)
inorganic
carbon
(TCO2)withthein-situdensity
(Ps,
r,P)and
the velocity (v) that is orthogonalto the section,from the
American(Am) to the African (Af) continentover the entire
water column(i.e., to the bottomdepth, - H). Becausewe
haddiscretestationspacing,the integralwasreplacedwith a
sum over station pairs. It was assumedthat the oceanis in
geostrophic
balanceexceptfor a directlywind-drivenEkman
Residual
(lamol
kg-1)
-20
-10
0
10
20
0
10
20
velocity.Two alternativeapproaches
to definingthevelocity
fieldwereused:a level-of-no-motion
approach
andaninverse
model approach.
1000
-
3.1. Level-of-No-Motion Approach
For computational reasons associatedwith different
constraintsused for the calculationof heat tramport (which
assumeszero net mass transport across a section) and
salt/carbon/mass
transport(in whicha nonzerosalttransport
constraintis invoked),thevelocityfield wasbrokendowninto
several components.This decompositionalso facilitated
sensitivitystudiesthat examinedsourcesof uncertaintyin the
inorganic carbon transport estimates.The velocity at any
2000II
3000 -
location on a section was
II
v(x,z)
=v/(x,z)
+[v•r(x,z)
- •r ]
-¾'comp
4000
-
5000
-
(3)
- ¾'comp + ¾
where v'(x,z) is the "baroclinic"velocity which can be
calculateddirectlyfrom thehydrographic
dataandis defined
so that for eachstationpair
6000
0
f
-H
az=o
(4)
andve•:is theEkmanlayervelocity
for eachstation
pair,
whichwasassumed
tobedistributed
evenlythrough
theupper
50 m of the water column and to be zero below that. It was
-20
-10
Figure 2. The differencebetweentheTCO2concentration
as
measuredon water samplescollectedduringMeteor 15 (19 oS)
and that predicted using 'the multiple linear regression
approachdescribedin thetextfor step1 of thedatainterpolation procedure.For this cruise, missingnutrientdata were
first interpolatedusinga multiple linear regressionusinga
calculatedfrom HellermannandRosenstein's
[1983] climatological averagewind stressdata. The integratedEkman
transportoverthe entiresectionwasthenmasscompensated horizontal
domainof 2 øx2 o. Therelativelysparse
TCO2data
by a spatiallyuniform,depth-independent
velocityin order setfor thiscruiserequireda largerinterpolation
domainof 5 o
that this componentof the transportgave zero net mass x5 ø.
HOLFORT
ET AL.' SOUTHATLANTICCO2TRANSPORT
useof a fixed horizontalpressurelevel for the entiresection
(we usedlevels which were varied systematicallyevery 400
dbarbetweenthe surfaceand 5200 m); (2) selectionof certain
key isopycnals
andisotherms;
and(3) useof a geographically
variable
level
of no motion
derived
from
a water
mass
analysis.Oneparticularspecification
of thistypewasusedfor
severalsubsequent
calculations
andis referredto as LNM-2.
Detailsconcerningthis specificationare givenby Holfort et
nfo
483
nfo
-S
Am -H
Am -H
Af O
Am -H
Af O
al. (manuscript
inpreparation,
1998).The ½•vM
fieldderived
from the level-of-no-motionchoicewassubsequently
adjusted
in orderthattherebe zero netmasstransportacrossthe entire
sectionarising from this componentof the transport.This
(7)
+ f f•œNM.S.@s,T,?
dxdz
"compensating"
velocitywasdefinedas
Am -H
Af0
- LNM
Af0
Am -H
V'cømp
=
AfO
(6)
Am -H
As with the Ekman transportmasscompensation,
this
adjustmentvelocity was distributeduniformly acrossthe
entire section.Note that the barotropicmass transport
compensation
requiredto satisfythezero-net-mass
transport
constraintcould be quite large. However, when this was
distributed
uniformlyacrossanentiresection,thelargecrosssectionalarea involved(sectionN5000km wide by N5 km
deep) implied that the corresponding
barotropicvelocity
adjustments
were too smallto be directlymeasurable(of the
where •s is the salttransportthroughthe BeringStrait.
3.2. Inverse Model Approach
In order to reduce the uncertainties and avoid some
arbitrariness associated with the choice of a level of no
motion,we alsodevelopedan inversemodelusingadditional
constraints
to assistwith the specification
of the barotropic
velocityfield. Note thatwith thisapproach,we solvedfor the
entirebarotropicvelocityfor eachstationpair, andtherewas
orderof 0.001 m s-•).
no artificialdivisionintothethreedistinctbarotropiccompoFinally, an extra, spatiallyuniform,barotropicvelocity nents used with the level-of-no-motionapproach.In the
component,
3-s, wasspecified
in orderto forcethenetsalt inverseanalysis,the constraintsare usedto set up a linear
transport
acrossthe sectionto matchthenettransportof salt equationsystemor "model"for the unknownvelocities,and
whichenterstheAtlanticthroughtheBeringStrait.Evapora- the systemwas thensolvedusingsingularvaluedecomposition, precipitation,andrunoff,because
theyinvolvefreshwa- tion. This methodwas usedby Wunsch[1978] in the North
ter, have a negligible effect on the mass of salt contained Atlantic, and calculationsusing South Atlantic data were
withintheocean(eventhoughsuchprocesses
alterthesalinity performedby Fu [1981], MacDonald [1993], Holfort [1994]
via dilutionand concentration).
Therefore,if a steadystate and Holfort et al. (manuscriptin preparation,1998). We
refer the readerto thesepublications
for detaileddescriptions
salt contentof the Atlantic Oceanis beingmaintained,the
amount of salt which enters the Atlantic Ocean to the north
of the inverse method; a listing of the constraintsthat we
throughthe Bering Strait must be balancedby the same employedis given in the appendix.
In performingthe inverseanalysis,solutionsbasedupon
amountof saltleavingthe Atlanticto the south.Given a mass
transport
of 0.8 x 109kgs-i [Coachman
andAagaard,1988] many differentmodelswere explored;thesedifferentmodels
anda meansalinityof 32.5, thesalttransport
throughBering included different formulationsfor the weighting of the
Straitis approximately
26.7 x 106kg s-1. This mustbe additionalconstraintsand data as well as the three separate
variantsof a phosphate
transportconstraint(seeappendix).
balancedby the meridionalsalt transportacrossour zonal
sections
in the SouthAtlantic.While therequiredcomponent The sensitivityof the solutionsto the initial specification
of a
of thebarotropictransport
isvery importantfor theoverallnet level of no motion was also explored.The goal of suchan
carbon
transport
across
anoceansection(upto 1 x 109kgs-1, approachis to find a setof solutionsthatis consistent
with the
carrying
upto2000kmols-i;seebelow),it does
notnecessar- constraintsandthe datato within a specifiedstringency.An
ily contributeto a carbon tramport divergencebetween exact solutionto the imposedconstraintsis generallynot
sections.
As a result,it hasoftennotbeenexplicitlyincluded desired because the constraints are themselves not an exact
of the ocean'sbehaviorandhaveuncertainties,
in prior calculations
of the carbontransport.Someconfusion representation
has arisen in the literature as a result of this distinction, as do the data. At high matrix rank, which corresponds
to a
however(see section7). This "extra"barotropictransport stringent
specification
of the constraints,
sucherrorsand
component,whichwas assumedto be distributeduniformly uncertaintiestendtb be magnified,resultingin tinreasonably
acrossan entire section,resultedin a net masstransport high andvariablebarotropicvelocities.At low matrix rank,
across
eachof thesections.
Theassociated
velocity,3-s,was the solutionwill be out of the range of the uncertaintyof the
calculated from
data and/or constraints, and the solution therefore can be
484
HOLFORT ET AL.' SOUTH ATLANTIC CO2 TRANSPORT
judgedincorrect.Our goalwasto f'mda setof solutions
that
sophisticated
LNM choice(LNM-2) gavea somewhat
larger
satisfiedthe constraintswithin reasonableerror margins.It
shouldthereforebe rememberedthat there is no uniqueor
"correct" solution, but there is rather a range of different
solutionsthat dependon the rank and the particularsof the
model including its initial state and factors such as the
weightingandscalingof thevariousequations
andunknowns.
valuefor Tc of-2345 kmols4 (Table3). A difference
betweenthisvalueand Tc using the "baroclinic"choiceof
LNM (Table3) of-442 kmols-1reflectsa smalladditional
barotropic
contribution
to thenetcarbontransport
(TcmU).
Henceat 11øS,T•c andT••:arebothsignificant
butcanbe
viewedasroughlybalancingeachothersothatthe net carbon
transportTc is carriedprimarily by the net masstransport
4.
Results
across
thesection
(TSc).
and Uncertainties
At 30øS, the Ekman masstransportis muchsmaller,and
4.1. Overview of the Inorganic Carbon Transport
T••:isa negligible
+ 35kmols-• ofinorganic
carbon
(northward).At thislatitude,
theTc
swas(-0.53x 109kgs'• x 2189
In order to set the contextfor subsequent
discussionof
•mol kg4) or -1160 kmol s'• of inorganic
carbon.The
uncertaintiesin the carbon transport estimates,we first baroclinic
component,
T•c, wastherefore
(-3758- 35 +
summarizetheresultsfrom our analysisof thedata.Through- 1160)or -2633kmols4. Useof themoresophisticated
LNM
out we use the conventionthat positive numbersrefer to
(LNM-2, Table3) reducedtheoverallTcvalueto -2720kmol
northwardtransport.In the following,T refersto transport s4 reflecting
an additional
barotropic
component
(T• M)of
estimates
(with Tc referringto carbon,and T• referringto
+ 1038kmols-•. Henceatthislatitude,
themass-compensated
mass). C denotesthe concentrationof total dissolvedinorEkman(T•) andthe combined
barotropic
(r• M + Tc
s)
ganiccarbon.Superscripts
areusedto distinguish
thedifferent contributionsto the net carbontransportwere both small so
components
of the transport(seesection3)' EK refersto the
thatTc wasdominated
by thebarocliniccirculation
compomass-compensated
Ekman component,BC the baroclinic nent(T•C).
component,S refers to the net mass transportcomponent
With this approximatebreakdownin mind, we now
required to satisfy the salt transportconstraint,and LNM
examinethe uncertaintyin the carbontransportwhicharises
refers to the mass-compensated
barotropic component from uncertaintiesin the specification
of the differentformal
associatedwith the choiceof a level of no motion. Angled components
of the transport(mass-compensated
Ekman,
brackets(() denotean averagedquantity.Carbontransports baroclinic,netmasstransportaswell asthederailedspecificaarereported
in kmols'• (1 Gt C yr4 = 1 Pg C yr4 = 2642 tion of the barotropicvelocityfield).
kmols'•).
The overall best estimatefor the meridionalinorganic
carbontransport(Tc) in the SouthAtlanticOcean,between
11øSand 30øS,wasof the orderof-2150 lanol s4 (i.e.,
southward). This estimate, which was derived from the
inversemodelingapproach,appliesto Tc at the time of the
measurements:
theearly-to-middle1990s.Usingthelevel-ofno-motionapproachdescribedin section3, we canview the
Tc acrosseachsectionasbeingcarriedby thefourmainmass
transport
components
listedabove:/•c, /ea:,/•u, andif.
Usingthe resultsof the analysespresentedin Tables2, 3, and
4, it is possibleto distinguishtheir relative contributions
to
rc.
Table 2 indicatesthatat 11øS,therewasa largesouthward
Ekmanmasstransport
(T•e•:)persewithintheupper50 m of
4.2. Ekman Transport
For the purposeof illustratingpotentialerrorsassociated
withspecification
of theEkmantransport,weusethelevel-ofno-motionapproachwhich views the mass-compensating
transportasa zonallyuniformbarotropicmasstransport(see
section3). We note below that this approachmay give a
worst-caseview of the transportuncertaintiescomparedto,
for example,theinversemodelapproach.The mass-compensatedEkmancomponent
of the totalcarbontransportacross
thesection
(T••:) canbeapproximated
by
.(<C•/•>-<C>)
W = r;/EK
(8)
the water column, carryingthe TCO2 concentration
of this
upperlayer(Ce•:).The(southward)
carbon
transport
in this
layerwastherefore
verylarge(-12x 109kgs4 x 2055•mol
kg4) or -24,660lanols'•. Howeverthisis offsetby the
thatin ourfullcalculations,
weresolved
Ce•:andT•e•:foreach
northward mass-compensating
barotropic transportwhich
carries the higher concentrationcharacteristicof the water
columnaverage( < C > ). The compensating
transportcarded
stationpair.)
Table2 presents
valuesfor the variablesin (8) for the six
sectionsusedin this study.As notedearlier, the net Ekman
(+ 12 x 109 kg S'1X 2196•mol kg-•) or +26,352kmols4.
Hencethe net carbontransportattributableto thiscomponent
(Tb) wasdirected
northward
at + 1700kmols-•.
Table 3 presentsresultscalculatedusingthe "baroclinic"
ß_•
- 1 •UD
where< C> and< Ce•:>areaverage
TCO2concentrations
of theentiresectionandtheEkmanlayer, respectively.(Note
contribution
is approximately
+ 1700kmols-1at 11øSand
close to zero at 30øS. The concentrationdifferencein (8)
remains
atabout140ttmolkg4 forall of thesections,
andthe
variation
in Tc
ex fromonesection
to another
is therefore
almostexclusivelya timarianof t2neE'._•2v.
mnmasstramport
s4 at 11øSincludes
thecontribution
fromTc
exdiscussed
inthe (T•eg, seeTable2).
Thereisconsiderable
uncertaintyconcerning
themagnitude
previous
paragraph,
aswellasfromTc
s. Thelattercarries
(-0.92 x 109kg s-• x 2196 •mol kg-•) or -2020kmol s4
of T•eg arisingfromuncertainties
in windstress
data.For
(Tables 2 and 4). Hence the transportof carbonassociated comparison,thelargestdifferencesof meanzonalwind stress
withthebaroclinic
structure
ofthevelocity
field(7•c)at11øS in the North Atlantic between the data sets of Hellermann and
is (-1903- 1700+2020) or -1583kmols-•. Useof a more Rosenstein[1983] and Isemer and Hasse [1985] are of the
HOLFORTET AL.' SOUTHATLANTICCO:TRANSPORT
485
Table2. Values
oftheMass-Compensated
Ekman
MassTransport
(T•K), theMeanTCO2Concentration
fortheEkman
Layer( < C> eK)andtheEntireSection
( < C> ), andtheNetCarbon
Transport
Associated
withtheMass-Compensated
Ekman
Transport
(Tong)
forSections
atVarious
Latitudes.
Cruise
Latitude,
T •s:
deg
S
109•g's
'l
< C> •s:,
< C> ,
/zmol
kg
'l
Tc•c
/zmol
kg
'l
kmol
•-I
Meteor 28
II
-11.8
2057
2196
+ 16,/..
Oceanus 133
11
-12.0
2053
2196
4-1744.
Meteor 15
19
-6.3
2071
2193
+785.
Oceanus 133
23
-3.8
2057
2188
+ 503.
SAVE Leg 3/4
25
-1.5
2047
2190
4-231.
30
-0.4
2046
2189
+35.
Meteor
22
Transports
are positivenorthward.
Table 3. Sensitivity
of CarbonTransportEstimates
to Uncertainties
in EkmanTransportandto the SpatialResolution
of the Section Data.
ModelParameters
Choice
ofLevelof
NoMotion
Transports
(Tc), kmols'l
Resolution
HEK
m
zadjust
Meteor
11øS28
Vertical
Meteor
19øS 15
Meteor
30øS 22
Horizontal
Uncertaintiesin EkrnanTransport
Baroclinic
50
-
1
1
-1903
-2741
-3758
Baroclinlc
20
-
1
1
- 1903
-2734
-3761
Baroclinic
100
-
1
1
-2045
-2793
-3762
Baroclinic
50
-0.2
1
1
-989
-2223
-3417
Barochnlc
50
+0.2
1
1
-2816
-3259
-4100
Barocllmc
50
SOC
1
1
-2594
-3026
-3793
SpatialResolutionof the SectionData
Bar oclinic
50
-
1
1
- 1903
-2741
-3758
Baroclinic
50
-
1
0.5
-2034
-2862
-3805
LNM-2
50
-
1
1
-2345
-2317
-2720
LNM-2
50
-
1
0.5
-2423
-2562
-3162
LNM-2
50
-
NODC
1
-2186
-2932
-2884
LNM-2
50
-
NODC
0.5
-2151
-2956
-3620
LNM-2
50
-
Met25
1
-2545
-2865
-2838
LNM-2
50
-
Met25
0.5
-2574
-3110
-3302
LNM-2
50
-
Met25
0.33
-3030
-3642
3545
HEI•istheassumed
depth
oftheEkman
layer;"zadjust"
refers
toadjustments
(dynes
cm':)totheHellermann
andRosenstein
[1983]
wind stressdata; "SOC" refersto the altenativewind stressclimatologyfrom the Southampton
Oceanography
Center (seetext);
horizontalresolutions
of 0.5 and0.33 referto subsampling
of sections
usingeverysecond
andeverythirdstation,respectively.
Results
for three differentverticalresolutions
are presented:1 (full 2 m resolution),NODC (standardbottle depthsof the National
Oceanographic
DataCenterasof 1988),andMet25(standard
bottledepths
usedduringthehistoric
Meteorexpeditions
of 1925and
1927).
486
HOLFORT
ETAL.' SOUTHATLANTICCO2TRANSPORT
orderof 30% (i.e., 0.3 dyncm'2at 15øN).Onthebasisof the
NorthAtlanticcomparison,
we estimated
theuncertainty
in
T• using
anassumed
errorinwindstress
of + 0.2dyncm'2.
Examplesof the sensitivityof the totalcarbontransportto
this uncertaintyfor three of the sectionsare presentedin
Table3. T•e•: is a fimcfion
of thewindstress
0:) andthe
Coriolisparameter:
months,
andweusedclimatological
average
windstress
data
to estimatethe Ekmantransport.Our calculations
therefore
implicitlyignore(or average)seasonal
factorsthat might
affect the carbon transport.Using data collectedat the
BermudaAtlanticTime SeriesSiteas an example,seasonal
changes
of TCO2concentration
of theorderof 40 •tmolkg'l
canoccurin theoligotrophic
oceanwiththisvariabilitymost
pronouncedin the upper 50 m - 75 m of the water column
S
(9)
[Bateset al., 1996]. At 11øS, wherethe surfacelayer mass
transport
is strongest
(about10x 109kg s'l) dueto strong
Ekmantransport,variabilityof thismagnitude
couldtranslate
Hence for a given uncertaintyin wind stress(r), the
uncertaintyin Tc is muchgreaterat 11øS (+50% or 900
into seasonal
Tc variabilityof the orderof 400 kmol s'l.
Variability at the higher-latitudesectionswould be smaller
than this because of the smaller Ekman contribution to the
kmols'i, Table3) thanat30øS(+ 10%or400kmols'l, Table transport.
However,in thissimplecalculation
weareignoring
3) asa resultof thevariationof theCoriolisparameterfwith possible
seasonal
covariance
of upperlayerTCO2concentralatitude.Useof an alternative
windstressclimatology
from tionswith theEkmantransportaswell asanytimedepend-
the Southampton
Oceanography
Center[Joseyet al., 1996; enceof thenon-Ekmantransport.Obviously,we havea lack
(seealsowww.soc.soton.ac.uk/JRD/MET/fluxclimatology.
of data with which to reliably assesssuchseasonaleffects,
html) S. A. Joseyet al., New insightsinto the oceanheat andthiswill be an importantissuefor furtherdatacollection
budgetclosureproblemfromanalysisof theSOCair-seaflux
andmodelingstudies.
climatology,
submitted
to Journalof Climate,1998]givesa
southward
inorganiccarbontransportthatis 691 kmol s-1
greaterat 11øS,withthedifference
decreasing
to 35kmols'l
at 30øS (Table 3).
4.4.
Data Resolution
It is worthnotingthatwhereascarbontransports
basedon
thelevel-of-no-motion
approach
appearverysemifiveto wind
stressdatauncertainty,
thismaybe anartifactof thesimple
approach
usedtospecifythecompensating
barotropic
velocity
field. The sensitivityto variationsof wind stressof the full
inverse
modelresults
wasconsiderably
smaller.Forexample,
at 11øS,theinversemodelcarbontransport
variedby only
about200-300 kmol s-1for the variouswind stressscenarios
given in Table 3. The reasonfor the reducedsensitivity
presumablylies in the details of how the inversemodel
distributesthe transportrequiredto masscompensate
the
Elanantransport.
Anotherpossiblesourceof uncertaintyis the choiceof
secondand every third station(resolutionfactorsof 0.5 and
0.33, respectively).We alsoexaminedthe effectsof vertical
Elananlayerdepth(/_/e•:),
whichinfluences
thevalue of
data resolution and combined vertical and horizontal resolu-
Accurate
calculations
of Tc fromzonalsections
require
veryclosestation
spacing,
because
sums
overstation
pairsare
substituted
fortheintegralin (2). We cannot
directlyestimate
the error associated
with thissubstitution.
However,with a
mean stationspacingof -•50 km, generallycloserat the
boundariesandwider in the oceaninterior, themain scalesof
motionsshouldbe resolvedby our data(thiswasnot neces-
sarilythe casewith someearlierstudies;seesection7). In
order to providean indicationof the possibleerror, we
"artificially"'subsampled
threeof thesections
usingevery
(C e•:) in (3). Normally,
we usethemeanTCO2of the tion by subsamplingthe full sectiondata at standardor
upper50 dbarof thewatercolumnastheTCO2concentration traditionalbottlesamplingdepths.Two variantsof standard
for the Elanan flow; however, we also made calculations depths
wereused:thestandard
bottledepths
usedby theU.S.
National OceanographicData Center as of 1988 and the
using meansof the upper 20 and 100 dbar to assessthe
sensitivityof the carbontransportto thischoice.The results standarddepthsemployedon the historicMeteor cruisesof
presented
in Table 3 suggestthat the overalluncertainty 1925 and 1927. In all thesereducedresolutioncases, the
associated
(vithchoosing
different
Elanandepths
is small entirevelocityfield wasrecalculatedbasedon the "artificial"
( • 100kmols'l) because
thechange
of ( Ce•:) withvarying dataset, andthe propertytransportswere recalculated.
Ekmandepthis small.Again,theeffectis negligible
at 30øS
dueto the low Elananmasstransportat thatlatitude.Similarly,theuncertainties
associated
withanuncertainty
of about
The resultsare also presentedin Table 3. The effect of
halvedhorizontalresolutionappearsto be relativelyminor
layer (seeFigure 2) are small.
decreasedvertical resolutionalso varied betweensections,
(change
in Tcof -•160kmols'l),although
a largerdifference
5 t•molkg'l inthemapping
oftheTCO2values
inthesurface at 30øSwasobservedusingtheLNM-2 model.The effectof
largeeffect(-•600kmols-1)being
observed
Thepotentially
largeuncertainties
associated
withspecify- withaparticularly
at 19øSbut with effectsat theothersections
beingonly 100ing the Elanantransportimply that an importanttaskfor
it canbeseen
that,depending
onthe
improvingtransportcalculations,
not only for innrg•nic 200kmols'l. In general,
section,bothhorizontalandverticaldataresolution
changes
carbonbut also for heat, nutrients,etc., is to obtainbetter
can affect the inorganiccarbontransportestimates.Table 3
estimates
of thewindstress,particularlyat low latitudes.
shows that the combined effect of decreased vertical and
4.3. Seasonality
horizontalresolutions
canbe quitesignificant,
withchanges
of 700-1300lanols-1 (againdependent
on theparticular
The sectionswere collectedprimarilyduringsummer sectionexamined).
487
HOLFORT ET AL.' SOUTH ATLANTIC CO2TRANSPORT
AtlanticDeep Water (02 > 37.07 and 04 < 45.92) flowed
southward;
(3) theflow of upperNorthAtlanticDeepWater
4.5. Barotropic Component
A measureof theuncemintyin Tc thatcanariseasa result
of uncert•ntyin thespecification
of thebarotropiccomponent
of thevelocitycanbe obtainedfrom exploringthe sensitivity
of Tc estimates
to differentchoicesof a level of no motion
(LNM). As describedin section3, we employed35 different
LNM
choices. Some of the choices resulted in meridional
overturning circulationsthat are incompatiblewith our
knowledgeof theoceancirculation.We thereforedefinedthe
circulationassociatedwith a particularLNM choiceto be
"valid"if the followingcriteriaconcerningthe circulationof
major water masseswere met: (1) the flow of Antarctic
BottomWater (o4 > 45.92) wasnorthward;(2) lowerNorth
waslessthan26 x 109kgs-l;and(5) theAntarctic
IntermediateWater (26.8 < o0 < 27.4) flowednorthward.
Thefrequencydistributions
of Tc estimates
resultingfrom
theLNM choices
thatgave"valid"circulations
arepresented
for thedifferentsections
as solidbarsin Figure3; theopen
bars representtransport estimatesassociatedwith LNM
choicesthat gave "invalid"circulations.The meanvaluesof
Tc for the "valid"choices(Table4) lie between~2000 kmol
s-1and~3500kmols'1to thesouth,andthemedianvaluesdo
not differ very much from thesemean values.The standard
Meteor 22; 30øS
Meteor 15; 19øS
Meteor 28; 11øS
I
16
14
o
(02> 36.9and02< 37.07)wasatleast5 x 109kgs'1tothe
south;(4) the totaltransportof North AtlanticDeepWater
I
Valid
[•1
Invalid
I
I
I
I
I
I
I
I
16
C
A
12
12
lO
10
8
ß-
6
6
4
4
I
......
0
0
0
I
......
0
0
0
0
I
0
0
0
0
0
0
....
I
0
I
0
0
0
FI o
0
0
0
TCO2Transport
(kmols'•)Southward
Oceanus 11øS
SAVE 25øS
Oceanus 23øS
'
16
'
I
I
i
i
I
• 16
Valid
D
14
14
Invalid
12
12
o
>' lO
10
8
ß
•
6
6
4
4
2
0
0
n ....... o,,
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
!
I-I 0
,
o
o
o
,•.
TCO2Transport
(kmols'•)Southward
Figure3. Frequency
distributions
oftheTc (inkmols'• onthex axis)forthesixdifferent
zonalsections
based
onthelevel-ofno-motionapproach.Solidbarsrepresent"valid"choicesof a levelof no motion;openbarsrepresentlevelsof no motionthat
resultedin an "invalid"circulation(seetext for details).For thesesolutions,anydifferencebetweenthe salttransportacross
the sectionrequiredto balancethe salttransportthroughBeringStraitandthecalculated
salttransportbasedupona choiceof
a levelof no motionhasbeencompensated
for by an additional,horizontallyuniform,barotropicvelocity.
488
HOLFORT ET AL.' SOUTH ATLANTIC CO2TRANSPORT
deviations
rangebetween200 and650 kmols'• withthetotal
range(maximum-minimum)
for an individualsectionbeing
~1600kmols-•. Hencespecification
of a levelof nomotion
can be a very significantsourceof uncertaintyfor the Tc
estimates.
It wasin orderto reducethisuncertainty
thatwe developed
the inversemodel analysisdescribedin section3. Solutions
baseduponmanydifferentinversemodelswereexplored,and
the sensitivityof the solutions
to the initial specification
of a
level of no motion was also explored.A fairly consistent
pictureemerged:At low matrixrank,thesolutions
werestill
influencedsignificantlyby the particularchoiceof initial
LNM; however, at ranks of > 80 this influencelargely
disappeared.At ranks > 200, the exact natureof the constraintsbecameimportant,andthe effectsof modelanddata
un.certaintieswere amplified so that solutionsfor different
modelsdivergedsignificantly.There was a relativelystable
regionof carbontransportversusrank in the range 135 <
rank < 165. We chosethisrangeof solutionsasthebasisfor
the transportestimatespresentedin Table 4. This table
presentsmeantransportsfrom the inverseanalysistogether
withestimates
of thestandard
deviationacrossmultiplemodel
formulationsfor thisrangeof ranks.
In general, the transportsacross the various sections
derivedfrom the inversemodelapproachare lower thanthe
meanvaluesderivedfrom the level-of-no-motion
approach,
particularlytowardthe south. Examinationof the solutions
suggestedthat for the southernmost
sections,it was the
phosphate
transportconstraint
thatwasa principalreasonfor
the reducedtransportestimaterelativeto thosederivedusing
the level-of-no-motion approach. The inorganiccarbon
transport
wasquasi-linearly
relatedto thephosphate
tramport
with a slope(/iTc //iTj•o4) of 110-170,whereTj•o4is the
phosphatetramport. Overall, however, the uncertainty
associatedwith the barotropicvelocity specificationwas
reduced
through
useoftheinverse
modelto ~200kmols-•for
an individualsection,andtherewasalsoreducedvariability
between the means for the individual sections.
4.6. Net Mass and Salt Transport
In order to satisfythe constraintof a total southwardsalt
transport
of 26.7 x 106 kg s'• through
the sections,
our
calculationsrequirea net southwardmeridionalmasstram-
portof-0.96x 109kg s-1at 11øS,decreasing
to -0.53x 109
kgs'• at30øS(seeTable4). Thefreshwater
balance
(evaporation,precipitation,andrunoff) inferredfrom thisestimate
of the convergenceof the masstransportagreesvery well
with the completelyindependent
estimateof net freshwater
fluxesfor this latituderange(0.42 x 109kg s'l) by
BaumgartnerandReichel[1975]. As notedearlier,assigning
thisnet masstransportto a zonallyuniformbarotropicflow
givesan associated
inorganiccarbontransportof about-2000
kmol s'• at 11øS and -1160 kmol s4 at 30øS due to this
componentof the transportalone.
The uncertaintyof the Tc due to mostof the sourcesof
uncertaintyin thisnet masstransportis alreadyincorporated
in therangesof solutions
presented
in Table4. Thesedidnot,
however,considerthe uncertaintyin the specification
of the
salttransport
throughBeringStrait(andthereforethroughthe
SouthAtlanticsections).
Anuncertainty
of 0.2 x 109kgs-1or
HOLFORT
ET AL.' SOUTHATLANTICCO2TRANSPORT
25 % in the masstransportthroughBeringStraitresultsin an
uncertainty
of 6.5 x 106 kg s'l in thesalttransport
which
translatesinto an associatedadditionaluncertaintyof ~400
kmols4 forTcintheSouth
Atlantic.(Notethatsuchanerror
would be systematicacrossall SouthAtlantic sectionsand
therefore would not affect carbon transport divergence
estimates;seesection7.)
489
as a functionof the 19 separatedensityintervalsusedin the
inversemodel. The corresponding
densityintervalsare listed
in the Figure 4 caption, and their depth distributionsare
shownin sectionplots (Figure 5). A three-layersystemis
evidentin themasstransportdistribution,withpredominantly
northwardtransportin thelowerdensitylayers(ol < 32.00)
which include the Antarctic Intermediate Water, southward
transportin the middensitylayers(o• > 32.00 and o3 <
4.7. Structure of the Transport
41.53) which includethe North Atlantic Deep Water, and
The quasi-vertical
structureof the section-wide
transports
of mass,silicate,andTCO2 for theMeteor22 cruisealong
30øSare presentedfor onetypicalrealizationof the inverse
analysisin Figures4a - 4c). Note that at this latitude,the
Ekman transportis small and is includedin the uppermost
densitylayer. In theseplots,the totalnet transport.
is plotted
northward
transport
in thehighest-density
layers(o3 > 41.53)
which include the lower Circumpolar Deep Water and
Antarctic
The carbontransport(Figure4c) appearsqualitatively
very
similarindicatingthatthe overalldetailsandmagnitudeof the
inorganiccarbontransportare controlledto first order by
Mass
Transport
(109 kgs'1)
-6
-4
-2
i
I
0
2
4
I
I
Bottom Water.
Silicate
Transport
(kmol
s'1)
6-200
-100
0
i
.:,:.:.:.:.:.:.:.:.:,:.:.:.:.:.•
'..3
':3
.',.•1
:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.3
':;:;:;:;:;3
:,:.:.:.:,:,:,:.:,:.:.:,:.:.:,1
I;:;:;:;:;:;:
lO
i
300
400
i
0
I.:-:.:.:.:.:,:.:,:.:,:.:,:.:.:
F:.:.:.:.:.:.:,:,:.:.:.:,
lO
...
::::::::::::::::::::::::::::::::::
1:;:;:;:;5:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:
(1)
i:;:;:;:;:;:;:;:;:;:;:;:;:;:;1;:;:;:,
•;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;
i:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:,:.:.:.:.:
t.:.:.:.:.:,:.:
ß!:
200
I:;:;:;:;:;:;:;:,
F:,:.:,:.:.:.:.:.:.:.:.:.:.:,
•.:.:.:-:.:.:.:.:,:.:.:t:.:•:,:•:
ß
100
i
;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;:;5:;:;:;:;:;:;:;$
i.:.:.:,:.:.:,
15
-
15
:.:.:.:.:.:4
o
'!:
o
;:;.;:;.;:;:;:;1;:;.;.;.;:;..;:;:;:•
B
I
20
:.:.:.:.:,:.:.:.:.:.:•.:.:.:.:.:4
I
I
I
5:;:;:;:;:;:;:;:;:;:;!
.:.:.:.:.:.:,:.:,:.:.:.:,:.:.:.:.:,:,:,:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.
I
I
I
!
20
0
0
ß
lO
lO
(•
ß!:
15
15
'!:
o
o
20
20
-8000
-4000
0
4000
8000
InorganicCarbonTransport
(kmol
s'1)
-20
0
20
40
60
80
100
Anthro.
CO2 Transport
(krnol
s-1)
Figure4. Verticalstructure
of thetransport
of (a)mass,(b)silicate,
(c)TCO2,and(d)anthropogenic
CO2at30øS(Meteor22)
as derivedfrom a typicalinversemodelsolution.Northwardtransportis positive.The verticalaxis givesthe densitylayer
numberfrom theinversemodel.The locationsof thelayersare shownin sectionplotsin Figure5 andweredefinedasfollows:
1, o0 < 26.60; 2, 26.60 < o0 < 26.80; 3, 26.80 < o0 < 27.00;4, 27.00 < o0 < 27.20; 5, 27.20 < o0 ando• < 32.00;
6, 32.00 < o• < 32.16;7, 32.16 < o• ando2 < 36.82;8, 36.82 < o2 < 36.92; 9, 36.92 < o2 < 36.97;10, 36.97 < o2
< 37.00; 11, 37.00 < o2 < 37.02; 12, 37.02 < o2 < 37.04; 13, 37.04 < o2 ando3 < 41.50; 14, 41.50 < o3 < 41.53;
15, 41.53 < o3 ando4 < 45.93; 16, 45.93 < o4 < 45.96; 17, 45.96 < o4 < 46.00; 18, 46.00 < o4 < 46.02; and19, o4
> 46.02
490
HOLFORTET AL.' SOUTHATLANTIC CO2TRANSPORT
__
ooo
•--- 32.00
•
1000:,/•;.v •
2.'7.
U"-•
ooo
I ' 56.9'/•
2000
3000
ooo
4OOO
000
5000
000
.•.50
45.93
000
6OOO
-30
-20
-10
0
-30
10
-2_0
-10
0
10
longitude
longitude
½ Ob
....... , ......... ' ......... , ......... "&6:8"-.-:-•
1000
2000
3000
4OOO
5000
6000
-30
-20
-10
0
10
-40
longitude
-30
-20
-10
0
10
longitude
Figure5. Distribution
of the19layersusedfor theinverse
modelanalysis
in fourof thesections
usedfor thisstudy.(a)
Oceanus
133at 11øS;(b)Meteor28 (WOCEA8) at 11øS;(c)Meteor15(WOCEA9) at 19øS;(d)Meteor22 (WOCEA10)
.at30øS.In allcases,
thelayernumber
wasassigned
sequentially
fromsurface
todeepaccording
tothedensity
criteria
listed
•n the captionto Figure 4.
variability in the mass transport and that concentration
variabilityplaysa smallerrole. Conversely,the influenceof
verticalconcentration
variabilityis very evidentin the caseof
the silicatetransport(Figure4b), wherethe effectof silicate
depletionin low-densitylayersandveryhighsilicateconcentrationsin the highest-density
layer canbe seento affectthe
overallpatternof the transportstrongly.
transportshoweda strong,northwardflowing subsurface
currentwhich corresponds
well with the combinedacoustic
dopplercurrentprofiler / CTD observations
of $chottet al.
[1995]. The solution also revealed a southward flow of
AntarcticIntermediate
Wateralongthewesternboundaryat
30øS,asobservedfrommooringsandfloatstudies[Boebelet
al. , 1997].
As with the vertical structure,the horizontal structureof
the inorganiccarbontransportis dependenton the mass
transportdistribution.The horizontalmasstransportderived
from the inversemodelanalysisis a complexsubjectwell
beyondthe scopeof this paper:it is coveredin detailby
Holfort[1994]andHolfortet al. (manuscript
in preparation,
1998).However,theinferredcirculationpatternwasfoundto
be broadlysimilar to that of Reid [1989], althoughsome
differenceswere alsoevident.At the westernboundary,the
southwardflowing Brasil Current at 30øS (the southern
boundaryof the inversemodeldomain)was prescribedto
5. AnthropogenicCO2 and the Preindustrial
Transport
Theestimates
of Tcpresented
aboveareappropriate
forthe
periodof time duringwhichour TCO2 measurements
were
made, the early 1990s. However the concentrationof dis-
solvedinorganic
carbonin theoceanis increasing
withtime
asa resultof theuptakeof anthropogenic
CO2 derivedfrom
land use changesand fossilfuel burning.The near-surface
concentration
of TCO2atthislatituderangehasincreased
by
havea transport
of 10 x 109kg s-• (based
ondirectcurrent ~55/•molkg-• since
preindustrial
timesin response
to the
measurements).
At 11øS (our northernboundary),the mass increase
in atmospheric
(andnear-surface
ocean)
pCO2from
HOLFORTET AL.' SOUTHATLANTICCO2TRANSPORT
~275-280 •atm in 1700 [Neffel et al., 1985] to ~355-358
•atm for the time of the WOCE surveys[Conwayet al.,
1994]. It is worth consideringwhatthe oceanictransportof
inorganiccarbon within our study region would have been
duringpreindustrial
times.In doingthis,we implicitlyassume
that oceancirculationpatternsand rateshave not changed
significantlysince1750. (We notethatobservations
of longterm changesof water propertiesat variouslocationsin the
oceanassociated
with changingatmosphericforcing [e.g.,
Dickson et al., 1997] may prove difficult to reconcilewith
this assumption.)
In orderto estimateTc for theperiodprior to theintroductionof anthropogenic
CO2 (i.e., prior to about1750), we first
estimated
the anthropogenic
CO2 contentof our TCO2 data.
We used a variant of calculationprocedures,basedupon
preformedCO2, whichwereintroduced
byBrewer[1978]and
491
early1990s.Thetransport
of anthropogenic
CO2to thenorth
arisesbecauseit is theupperoceanwaterswithinthislatitude
rangethatcontainmostof theanthropogenic
CO2 , andthese
are moving predominantlynorthward(Figure 5d). Conversely,the southward
returnflow of "older"deeperwater
contains
a muchlowerconcentration
of anthropogenic
CO2.
This supplyof anthropogenic
CO2 to the northernand
tropicalAtlantic Ocean which is carried by the ocean's
meridional
overturning
circulation
isquantitatively
significant
relativeto theuptakeof anthropogenic
CO2acrosstheair-sea
interfacewithinthe NorthAtlantic.For example,Sarmiento
et al. [1995]reporteda model-derived
estimateof theuptake
of anthropogenic
CO2 dueto air-seagasexchangewithinthe
northernAtlantic (18øS - 78øN) of-1000 kmol s-•. Our
estimateof the transportof this propertynorthwardacross
20øSwithinthe oceanapproaches
50% of thisvalue.We will
Chen and Millero [1979] and included the contribution of
discussthe importanceof this meridionaltransportfor the
nitrate remineralizationto alkalinitychanges[e.g., Brewer overallbudgetof anthropogenic
CO2withintheNorthAtlantic
and Goldman,1976]. It is beyondthe scopeof thispaperto elsewhere.
discussthecalculationprocedurein detail;however,it shared
Subtraction
of theanthropogenic
carbontransport
fromTc
severalsimilaritieswith an approachemployingtransient allowsestimationof the inorganiccarbontransportduring
tracer data developedby Gruber et al. [1996]. The estima- preindustrial
times(Table4). The transportacross10øSwas
tionof theanthropogenic
CO2concentration
doesnotcontrib- about-2900kmol s-• (i.e., to the south),whichcan be
ute greatlyto the overalluncertaintyof eitherthe anthropo- comparedwith the estimatedpreindustrialtramportinto the
genicor preindustrialcarbontransportsrelativeto uncertain- Atlantic via the Bering Strait of about 1630 kmol s-•
tiesassociated
with thecirculationandmasstransports.
[Lundbergand Haugan, 1996]. There thereforeappearsto
The averagedepthprofileof anthropogenic
CO2for each havebeena preindustrial
(natural)divergence
of theinorganic
of the sections
is presentedin Figure6. Very noticeablein all
of theprofilesis a relativelyhighanduniformanthropogenic
Anthropogenic
CO2 (lamol
kg-1)
CO2 concentrationin the Antarctic IntermediateWater
betweendepthsof 500 m and1000m. Overall,theanthropo0
10
20
30
40
50
60
I
I
I
I
genicCO2 concentrations
in the water columnare highest
towardthe south,closerto the origin of the AntarcticIntermediateWater[cf. Brewer,1978].Our analysisgivesa small
averagebackground
levelof anthropogenic
CO2of 3-5 •mol
kg4 throughout
the watercolumnin eachsection.This
lOOO
nonzero background is consistentwith the detection of
significantlevels of the anthropogenic
halocarbonCC14
throughoutthe water column in the Brazil Basin and even in
portions of the deep Angola Basin [Wallace et al., 1994;
Roether and Putzka, 1996]. Carbon tetrachloride has been
presentin the environmentin significantquantifiessince
1920-1930at whichtime the anthropogenic
CO2 contentof
thecoldsurfaceseawaterwhichventilates
thedeepoceanwas
2OOO
3OOO
already12-14•mol kg-•. Theanthropogenic
CO2,having
beenintroducedsinceabout1750,mayhavepenetrated
farther
into the deepoceanthanCC14andconsiderably
fartherthan
the CFCs and bomb tracers such as 3H and •4C. Hence the
findingof CC14throughout
mostof thelow-to middle-latitude
SouthAtlanticbasinssuggests
thatno portionof the Atlantic
Oceancan safelybe assumedto be "free"of anthropogenic
CO2 contamination.
The anthropogenic
CO2 distributions
fromthe sectiondata
were combinedwith the masstransportcalculations
in order
tocalculate
thetransport
of anthropogenic
CO2. Theresulting
transportof anthropogenic
CO2wasconsistently
northward,
with a valuewhichrangedbetween400 and700 kmol s-• at
4OOO
,
5000
11os
---=--
19os
--•--
23os
-•--30os
.....* .... 11os (M28)
---',-'- 25os (SAVE)
6000
I
I
11øSandwas300 kmols-• at 30øS. The anthropogenicFigure 6. Averagedepthprofileof anthropogenic
CO2 for
carbontransportspresentedin Table 4 imply thatthe south-
thesections
usedduringthisstudy.Eachprofilerepresents
the
wardinorganic
carbon
transport
Tcwas~430kmols-•greater averageprofile for an entiresection. (Note thatmostindividacross20øSduringpreindustrial
timesthanit wasduringthe
ual profileshad bottomdepthsshallowerthan6000 m.)
492
HOLFORT ET AL.- SOUTH ATLANTIC CO2 TRANSPORT
Bering
90
90N
10S 30S
Bering
Eq õ60
Strait
Strait
Antarctica
10s 30s
90N
Bering
60S
1280'
/
650
Eq •
60s
Antarctica
o
90N
10S 30S
o
o
•
•
•
6OS
g
Unitsare kmol s4
(1 GT(C)yr4 = 2642kmols-')
Figure7. Schematic
summary
of theregionalAtlanticOcean
inorganic
carbonbudget(10øSto 30øS)basedon thewater
columntransport
andstorage
estimates.
(a) thecontemporary
budget,(b) thebudgetfor anthropogenic
CO2 , and(c) the
preindustrial
budget.Thetransport
andstorage
unitsarekmol
s4 . The number+240 in Figures7a and7b refersto the
storageterm. In Figure7c, the budgetis extendedto the
NorthAtlantic,andtheasterisked
numberat thetopboundary
refers to the combined effect of air-to-sea and rivefine
inorganiccarbonfluxes.
o
o
o
•
o
carbontransportbetweenBeringStraitand10øSof the order
of 1300kmols-1,whichwaslikelybalanced
bynetair-to-sea
transferof CO2 and riverineadditionsof inorganiccarbon
within the North Atlantic and Arctic Oceans(seeFigure 7c).
The potentialrole of organiccarbontransports
mustalsobe
considered
in assessing
thislarge-scaledivergence[Sarmiento
et al., 1995].
6. RegionalCarbon Budget(10oS - 30 oS)
The carbontransportversuslatitudedataof Table4 canbe
usedto examinethe regionalinorganiccarbonbudget.Figure
7 summarizes
the followingdiscussion
schematically
for the
contemporary,
1990sTCO2budget(Figure7a), theanthropo-
tt•
tt•
493
HOLFORT ET AL.' SOUTH ATLANTIC CO2TRANSPORT
genic
CO2 budget
(Figure
7b),andthepreindustrial
TCO2
section,
weestimated
theMtiDforanthropogenic
CO2tobe
770 m for the region between10øS and 30øS. The uncerA regression
of Tc versuslatitude(Figure8a)indicates
that taintyof thisestimatemay be of the orderof 20% [cf. Gruber
the contemporary(i.e., 1990s)inorganiccarbontransport et al., 1996]. Gammon et al. [1982] have shown that the
tracerwith a seadecreased
by 16.7 (_+19) kmols'• per degreetowardthe verticalprofile andMPD of a conservative
surfaceconcentration
thatis increasingexponentiallywith esouth,implyinga very slightconvergence
of 330 (+380)
kmol s'• between10øSand30øS(uncertainties
are standard foldingtime of t yearsreachesa "transientsteadystate"after
CO2,
errors).Any contemporary
convergence
of inorganiccarbon (3 x t) years [Gammonet al., 1982]. Anthropogenic
which has been increasingexponentiallysinceat least the
canbebalancedby a sea-to-airflux of CO2,netproduction
of
organiccarbon,net dissolution
of solidcarbonates,
andnet mid-1800swith t = ~30 years, shouldby now have achieved
storageof anthropogenic
CO2. (The regionalriver discharge transientsteadystate. Hence the storageor the inventory
asgivenbyPerryet al. [1996]is < 107kgs-• whichwould change of anthropogenicCO2 during the 1990s can be
carrya negligibleinorganiccarbontransportof < 10 kmol estimateddirectlyfrom thechangein themixed-layerconcenCO2 via (10). For the early 1990s,
s4.) Themagnitude
of theanthropogenic
carbon
storage
term trationof anthropogenic
of anthropogenic
CO2 wouldhave
can be estimatedfrom the observedmeanpenetrationdepth the surfaceconcentration
budget(Figure 7c).
(MPD) [Broeckeret al., 1979]for anthropogenic
CO2, which
is defamed as:
increased
by 0.7 to 0.8/•molkg4 yr-• in ordertokeeppace
withtheatmospheric
pCO2increase
of about1.2•atrnyr4.
This givesan averagelocalstoragerateof 0.59 (+20%) mol
m'2yr-•or240(+49) kmols-•whenintegrated
overtheentire
meo
=fc,
Atlantic Ocean surfacearea between10øS and 30øS (12.8 x
10•2m2).
whereC,aandCzaretheconcentrations
of anthropogenic
CO2
This estimate, which is based on the framework of a
vertical diffusion-advectionmodel [Gammon et al., 1982],
in the mixed layer and at depthz, respectively.Using the
ensemble
of individualanthropogenic
CO2profilesfrom each
can be tested against the directly observed buildup of
anthropogenic
CO• betweentheGeochemical
OceanSections
Latitude(South)
Latitude(South)
10
15
20
25
30
10
15
20
25
30
I
I
I
I
I
I
I
I
I
I
A.Contemporary
(1990s)
TCO
2
. Preindustrial
TCO
2
- -1500
-1000
-
-1500
-
-
-2000
-
- -2500
-2500
-
-
-3000
-3000
-
-
-3500
-2000
o
c-
'
I
I
-
- D.Dissolved
Oxygen
2000 B.Anthropogenic
TCO
2
0
1500
- -500
1000
- -1000
o
c-
o
500•
- -1500
0
-
I
I
I
I
I
10
15
20
25
30
Latitude(South)
10
15
20
25
c-
-2000
30
Latitude(South)
Figure 8. Mean transportversuslatitudeplots,with regression
lines and 95% confidence
intervals,basedon the results
presented
in Table4. Separate
plotsarepresented
for thefollowingparameters:
(a) thecontemporary
(1990s)inorganic
carbon
transport,
(b) theanthropogenic
carbontransport,
(c) thepreindustrial
inorganic
carbontransport,
and(d)thedissolved
oxygen
transport.
Notethatthetransport
axisforeachplotcovers
a totalrange
of2500lcmol
s4 andthattheconvention
usedissuch
thata negativetransportdenotestransportsouthward.
494
HOLFORT ET AL.' SOUTH ATLANTIC CO2 TRANSPORT
Study
expedition
oi•themid-1970s
andtheWOCEexpeditionscally driven fluxes of O2 and CO2 have oppositesigns,
of the 1990s. Wallace et al. [1996] conducted such an
analysis
for thewesternbasinof theSouthAtlantic(5ø- 35øS,
west of 20øW) using a multivariate,time seriesanalysis
technique
to comparetheTCO2 datacollectedduringthetwo
samplingperiods.(Notethatour datasuggest
thatthismore
localizedregionhasa meanpenetration
depththatis indistinguishable
fromthe 10ø- 30øSaverage.)On thebasisof this
empiricalapproach,the inventorychangeover 20 yearsis
estimated
to be 11.1 mol m-2whichgivesa storage
rateof
0.56molm-2, identical
to theone-dimensional
model-derived
estimate derived above.
whereasfluxesdrivenby thermalprocesses
operatein the
samedirection.In thisregard,our estimates
of themeridional
oxygentransport(Table 4 and Figure 8d) indicateconvergenceof the net southwardO2 tramport,whichwe assume
from direct transportestimates.In addition,the possible
effectsof seasonality
on the convergence
estimates
require
further examination.
7. PreviousCO2 Transport Estimates
There have been severalearlier publishedattemptsto
estimatethe meridionaltramportof inorganiccarbonin the
10øSand30øSof 330(_+380)kmolS'1in thecontemporaryAtlantic Ocean. These used several different data sets,
oceanis thereforesimilarin magnitudeto thisanthropogenic assumptions,
andcalculationprocedures
andare summarized
carbonstorageterm. Further, the difference (90 _+380 kmol
in Table5. A veryimportant
distinction
between
ourtransport
s'l) between
thecontemporary
inorganic
carbon
convergenceestimatesand many found in the earlier literature is our
andtheanthropogenic
carbonstorageis reasonably
consistent explicitincorporation
of the net meridionalmasstramports
with a completelyindependent
estimateof the net annualsea- requiredto satisfysalt and freshwaterbalances.$toll et al.
to-airfluxforthisregionof 264kmols-1derived
fromsurface [1996]notedthe significance
of the CO2 transport
contribupCO2 data[Takahashiet al., 1997]. The uncertainty
of the tionwhicharisesfromnetmeridional
masstransport
through
latterestimatecouldbe ashighas75 %. Thiscorrespondence Bering Strait. They also noted that changesin the mass
suggests
thatanyadditionalnetsourcesandsinksof inorganic transportas a functionof latitude,resultingfrom freshwater
carbonin thisregion,suchas the localnetproduction/respi- additionor removal, drive a significantmeridionalcarbon
rationof organiccarbonor dissolution
of calciumcarbonate, transportdivergence.
arenotsignificantly
different
thanzero(174_+400kmols-l).
FollowingLundbergand Haugan [1996] and $toll et al.
Figure 8b and Table 4 showthat during the 1990sthere [1996],we haveexplicitlyincludedthisnetmasstransport
in
wasa significant(p = 0.05) divergence
of theanthropogenic ourestimates
of themeridionalCO2transport.
ThiscontribuCO2 transportbetween10øSand30øSamounting
to ~320 tion wasnot, however,includedin the CO2 transport
esti(+ 128)kmols-1.Anair-to-sea
fluxof anthropogenic
CO2of matesgivenby Breweret al. [1989]andBroeckerandPeng
560 (_+137)kmol s'• wouldbe requiredto balancethe [1992]. It was alsonot incorporatedin transportestimates
combined anthropogeniccarbon storage and transport reportedin preliminarymeetingabstracts
by Holfort et al.
divergenceterms,assuming
no additionalsources
or sinks.In
[1994]andRobbins[1994]. The latterwascitedby Sarmiento
otherwords,the regionalsea-to-airflux of CO2 duringthe et al. [1995] in their estimationof the inorganiccarbon
1990swas approximately
560 kmol s'1 smallerthanthe transportdivergencebetweenBering Strait and the South
corresponding
sea-to-airflux of the preindustrialera.
Atlantic. Martel and Wunsch[1993] accountedfor runoff,
Subtraction
of theanthropogenic
carbontransportfromthe evaporation,and precipitationwithin their model domain
contemporary
transportimpliesa statistically
significant
(p =
(12ø-60øNin theAtlanticOcean)butapparently
chosenotto
0.1) localconvergence
of Tc between11øSand30øSfor the includethe salt and mass transportthroughtheir model
preindustrialera (Table4 andFigure8c). Duringthisperiod, domainarisingfrom the BeringStraitflow andrunoffnorth
we assumethatthe overallinorganiccarboninventorywasin of 60øN. KeelingandPeng[1995]accounted
for theBering
steadystate(zero storageterm), andif thebiologicalproduc- Straitcontribution
to theNorthAtlanticphosphate
balancebut
tivity of the regionwas comparableto thatof the contempo- chosenotto includeitscontribution
to thetotalmasstransport
rary ocean,the convergence
wouldhavebeenbalancedby a when calculatingthe inorganiccarbontramportacrossthe
considerable
netsea-to-air
fluxof CO2.Theaverage
preindus- equator.Carboncyclemodelsimulations
[e.g., Sarmiento
et
trial inorganic
carbonconvergence
(32.6 _+17 kmols'1per al., 1995] do not alwaysresolvethiscontributionbecausethe
degreeof latitude)impliesa totalconvergence
of 650 (_+340) transportthroughthe BeringStraitis not alwaysresolved.
kmol s-1 between10øS and 30øS. Alternatively,taking
To someextent,thechoiceof whetherto includeor ignore
Takahashi
et al.'s [1997]estimate
of 264 (_+198)kmols'1for this contributionto the net carbontransportis a matterof
the contemporarysea-to-airflux of CO2 (see above)and philosophy
andis contingent
on the specificquestions
being
adding
560(_+137)kmols'1to compensate
fortheair-to-sea askedof thetramportestimates.Oneof themajorreasons
for
transferof anthropogenic
CO2 impliesa preindustrial
sea-to- examiningcarbontransportdivergences
is to obtainestimates
air CO2 flux of 824 (_+240)kmols-1.Onceagain,thisis of the regionaldistributionof the net air-seaflux of carbon
closely comparableto the preindustrialinorganiccarbon (andoxygen).With thisspecificgoalin mind,it isjustifiable
convergence
suggesting
thatlocalnet production/respirationto ignorethe inorganiccarbontransportwhichis tied to the
of organiccarbonor dissolutionof calciumcarbonatewas net salt transportacrosssections.This componentof the
relativelysmallcomparedto thepreindustrial
sea-to-airflux. inorganiccarbontransport,while large, can be viewed as
Keelingand Peng [1995] haveshownthatthejoint inter- constant across all Atlantic sections and therefore does not
pretationof meridionaltransportsof both02 and CO2 can affector reflectthe air-seaflux of CO2 or 02. However,if
distinguish
thephysicalversusbiologicalmechanisms
respon- directcomparisons
are madewith the net transportof inorsiblefor the fluxesof thesegases.As theypointout, biologi- ganiccarbonthroughtheBeringStrait,suchasweremadeby
The very slightconvergenceof inorganiccarbonbetween
HOLFORT ET AL.: SOUTH ATLANTIC CO2 TRANSPORT
Sarmientaet al. [1995], then it is essentialthat the contribushouldnot have changedsince the preindustrialera. The
observation
thatthetransportof bothgasesin thepreindustrial
ocean was convergentis qualitatively consistentwith a
thermalforcingmechanism
drivingtheeffluxof both02 and
CO2acrossthe air-seainterfacein thisregion.
Our heattransportcalculations
(Table4) indicatesignificant (p < 0.1) divergenceof the northwardheat transport
between
10øSand30øSaveraging
0.012(_+0.006)x l015W
per degreeof latitude,implyingadditionof heatto the water
colunto. Watsonet al. [1995] have estimatedthe maximum
air-seaCO2 flux drivenby the heat transportof the North
495
precipitation,
whichcansignificantly
affectthedivergence
of
the net meridionalmasstransport[Stall et al., 1996] and
which mustbe accountedfor when estimatingcarbontrans-
portdivergences..As
notedbyStalletal. [1996],thepioneeringanalysis
byBreweretal. [1989]wasbasedonanassumption of zero net meridionalmasstransportand thereforedid
not include this effect.
Contraryto a statement
by Stall et al. [1996], the CO2
transport
estimates
of BraeckerandPeng[1992]andKeeling
and Peng [1995] do implicitlycorrectfor this effect by
normalizing
theobserved
inorganic
carbonconcentrations
to
constant
salinity.Throughthisnormalization,
theadditionof
Atlanticoceanto be ~0.75Gt C yr'• or ~2000kmols4 of
CO2uptakefor eachpetawatt
(10isW) of northward
heat
variable levels of freshwaterto different water masses(and
flux divergenceis harderto explainexceptas the resultof (1)
uncertaintyin thevarioustransportandconvergence
estimates
themselves(note the large error bars for the convergence
estimates
and,particularly,thepossibleinfluenceof seasonality of thetransports,section4.3); (2) violationof theassumption of no storagefor dissolvedoxygen;or (3) an "extra"
contribution
to a predominantly
thermallydrivenconvergence
arisingfrom somebiologicalmechanism(e.g., a divergence
of the organiccarbontransport).
On balance, the qualitativepicture is consistentwith
independent
estimates
of theair-seaCO2flux, andtheoverall
budgetfor preindustrialcarbonsuggests
a relativelyminor
role for biologicallydriven convergence/divergence
in this
particularregion.However,it shouldbe clearfrom theabove
that measurementsof the total organiccarbonconcentration
of seawater,which were unfortunatelynot availablefor the
sectionsused in this study, would provide independent
comtraintson the air-seaCO2 exchangescenariosinferred
tion of this componentof the transportbe includedin the
transportestimatesat all latitudes.
Of potentially more significanceis the effect of net
freshwateradditionor removalvia runoffor evaporation
and
netmass
flowthrough
a section
which
is'required
tobalance
hencedilutionof inorganiccarbon)is compensated
for. In
transport.Applyingtheir scalingto our observeddivergence principle,thiscorrection
shouldservethe samepurposeas
of the northward heat transportbetween 10øS and 30øS
keepingtrackof thefreshwater
transport
divergence;
how(Table4) givesa maximumthermallydrivenCO2sea-to-air ever, in practice,the efficacyof this "normalization"
apfluxof (2000x 20 x 0.012)or 480(_+240)kmols'l, which proachhasnotbeenindependently
verified.Thatis, asit was
is quiteconsistent
with the observedpreindustrialinorganic applied,did the salinitynormalization
reflecta reasonable
carbonconvergence
andestimated
preindustrial
sea-to-airflux
freshwater
transport
distribution?
In contrast,wehavechosen
of CO2 (see above). A similar calculationfor dissolved to explicitlyconsider
the salt,freshwater,
andcarbontransoxygen
givesa maximum
sea-to-air
02fluxof ~1500kmols-• portsratherthancalculating
thetransport
of salinity-normalPW-•ofheattransport
divergence,
whichimplies
a maximum ized quantities.Using a measurement-based
salt transport
constraint,estimatedbaroclinic, Ekman, and barotropic
thermallydrivensea-to-air02 flux of (1500 x 20 x 0.012) or
360 (_+ 180) kmol s-• for the samelatituderange.The transports,
andthe observed
salinitydistribution,
we were
observedconvergence
of oxygen,however,was1180(_+460)
ableto directlyestimate
thefreshwater
transport
divergence.
kmol s-• between 10øS and 30 øS.
This was consistentwith independentestimatesof this
Hencethe observedconvergence
of bothgasesis qualitaquantity[Baumgartner
andReichel,1975](seesection
4.4)
tively consistentwith the signof the observedheattransport therebyincreasing
our confidence
thatthe freshwater
transdivergence.In the caseof CO2,themagnitudeof the converportdivergence
is beingcorrectly
represented
whenestimatgenceis alsoquantitatively
consistent
with themaximumthat
ingtheinorganiccarbontransport
divergence.
wouldbe predictedtheoreticallyfrom the observedheatflux
Thisdiscussion
emphasizes
thewidevarietyof approaches
divergence, whereas for dissolvedoxygen the observed and philosophies
that havebeenpromulgated
in orderto
calculatecarbontransports.
Not surprisingly,
thisvarietyhas
convergence
is 2-3 timesthetheoretical
prediction.Giventhat
led to confusions
of interpretation
and a risk of comparing
air-seaequilibrationof O2 is an order of magnitudemore
unlike transportestimates. It is thereforeimportantto
rapidthanthat of CO2, it is perhapsnot surprisingthat gas
reiteratethat we have reportedthe total net transportof
exchangewouldcreatea strongerconvergence
signalfor 02
These
than for CO2. However, an observedoxygenconvergence inorganiccarbonacrossthe SouthAtlanticsections.
estimates
incorporate
thecarbontransport
associated
with a
larger than that predictedon the basisof the observedheat
the salt inputthroughBeringStrait. They thereforealso
reflect the meridionallyvaryingmasstransportwhich is
associatedwith freshwater addition and removal.
Table5 highlights
someotherwaysin whichourapproach
to calculating
inorganiccarbontransport
differsfromprior
estimates.Notably, our estimateis basedon a very large,
high-quality
dataset,includingfour sections
of TCO2data
which have been interpolatedonto the CTD data using
multipleregression.
Manyof theearlierestimates
werebased
on limiteddatasets,oftenrelyingon bulk averageconcentrationsfor differerawater massesor layers. Severalanalyses
havereliedon bulk overturningcalculations
in whichthenet
massfluxin anupperlayeranda deeplayeraremultiplied
by
zonallyaveraged
propertyconcentrations
for therespective
layers. The potentialrisksassociated
with estimatesthat are
basedon smoothed
fieldsare discussed
in detailby Rintoul
and Wunsch [1991], who found that accurate estimatesof
nutrienttransportscanrequirevery detailedestimatesof the
circulationandpropertyfields.
We examinedthe differencebetweenthe Tc calculated
usingour full inversemodel solutionsand that derivedfrom
496
HOLFORTET AL.' SOUTHATLANTIC CO2TRANSPORT
a zonallyaveragedestimateof thetransportfor theMeteor28
(11øS) and Meteor 22 (30øS) sections.For the zonally
averagedTc, we calculatedthe sum, over all 19 vertical
layers in the model, of the productof a layer-averagenet
masstransportanda layer-average
TCO2 concentration.
At
30øS, the zonally averagedsolutionwas only 100-250 kmol
andwas-2150_+200kmols'• across
20øS.Thistransport
includesa large contribution
associated
with a barotropic
velocityrequiredto satisfythe salt flux constraintmeasured
at the Bering Strait. While estimationof this contributionis
notessential
for assessing
theinorganiccarbonuptakeby the
NorthAtlanticoverturning
circulation
andwasnotincorpos4 (10%) differentfromthefullmodelsolution.
However,at ratedintoseveralearlierestimates
of oceancarbontransport,
11øSthezonallyaveraged
solution
wasalmost1000kmols-• it is criticalfor assessing
the oceancarbontransportdiveror 50% lower thanthe full solution.Inspectionof the solu- gencebetweenthe BeringStraitand the SouthAriantic.The
tionssuggested
thatthelargedifferenceat 11øSwastheresult principalsourcesof uncertainty
in the transportestimates
of a strongwesternboundarysubsurface
currentin thedepth originatewith the Ekman component
of the transport,
range 50-800 m at that latitude [cf. Schottet al., 1995] particularlyat low latitudes,togetherwith uncertaintiesin
coupled with a large difference in TCO2 concentration determining
the barotropic
component
of thevelocityfield.
between the western and eastern boundaries.
Thebarotropic
transport
uncertainty
wasreduced
through
use
We also comparedthe Tc derivedfrom the full inverse of aninversemodelandcouldprobably
bereduced
furtherby
modelsolutionswith that derivedfrom a two-layerzonally the use of more sophisticated
inversecalculations
and addiaveragedoverturningcalculationsimilar to that used by tional constraints.To lower the uncertaintyin the Ekman
Broeckerand Peng [1992] andKeelingandPeng[1995]. We
transport,a betterknowledgeof theannualwindstresswill be
definedthe upperlayer as o2 <36.8 and definedthe lower required.A better understandingof the effectsof seasonal
layer as o2 > 36.8. The magnitudeof the overturning variationsof the carbontransportis required.
circulationimpliedby thesecalculations
rangedfrom 11 to 14
By estimating
theanthropogenic
component
of theTCO2
x 10• kg s-1,whichis closeto thatassumed
byKeelingand distribution, it was shown that there was a net northward
Peng[1995]butmuchlowerthanthe20x 10• kgs-•assumed transportof anthropogenic
carbonacross20øS of 430 kmol
by Broeckerand Peng [1992]. As with the zonallyaveraged s-•.Thistramport
increased
toward
thenorth.Bydifference,
solutiondiscussedabove, at 30øS the simpleoverturning the preindustrialsouthwardtransportof inorganiccarbon
calculation
gavetransports
whichwereonly200-300kmols-•
across
20øSwas-2580kmols-• . Thepreindustrial
transports
(10-15%) lower than the full model solution.However, at
11øSthe overturningcalculationgavea transportwhichwas
shownetconvergence
of bothCO2and02 between10øSand
30øS, suggesting
that this region was a net sourceof both
700-900kmols-• or ~40%lowerthanthatderived
usingthe gasesfor theatmosphere
duringthepreindustrial
era, consisfull model. Obviously, caution must be exercisedwhen tent with thermalforcingof the gas flux. Our transport
attemptingto estimatethe transportof inorganiccarbon; estimates
alsosuggest
thattransportof anthropogenic
carbon
undercertaincircumstances
it appearsthatsimpleaveraging by the upperlimb of the meridionaloverturningcirculation
of the velocityand concentration
fieldscanprovideaccurate may supplya significant
fractionof the anthropogenic
CO2
carbontransports. However, in other circumstances
such
simplifiedcalculations
may be subjectto largeerrors.
Our analysissharesmanysimilaritieswiththework of $toll
et al. [1996], whoestimated
the CO2transportacrossa single
sectionbetweenGreenlandandIreland. Our analysisbenefits
from having been able to samplethe boundarycurrents,
whereasice precludedStoll et al. from samplingwithin the
East Greenland Current. Our multiple zonal sectionsalso
allow for a more detailedinverseanalysis.The SouthAtlantic
sectionsbenefit from being closed, whereas the section
occupiedby $toll et al. [1996] couldnot explicitlyaddress
transportvia the CanadianArctic Archipelago,the English
Channel, and the Irish Sea. For example, Lundbergand
Haugan [1996] estimatethe net masstransportthroughthe
Canadian
Archipelago
to be 1.4 x 10• kg s4 witha mean
which is currently being storedwithin the North Atlantic
Ocean.
Appendix
The constraints
usedin the inversemodelanalysiswereas
follows:
1. The total salttransportacrossthe SouthAtlanticzonal
sections
is26.7x 106kgs-•. Thisconstraint
isbased
onthe
salttransportthroughtheBeringStraitandtheassumption
of a steadystatesalt balancewithin the North Atlantic.
Notethatthisisa departure
fromthemorecommonly
used
constraint
of net masstransport
usedfor heattransport
calculations.
2. Salt is conservedin boxesboundedby the different
sections
and/orthecontinents
in thehorizontal
planeand
by specifieddensitysurfacesin the verticalplane.The
verticallayersincludethoseusedby MacDonald[1993]
TCO
2concentration
of~2070
•tmol
kg-•s._•
This
implies
a
nonnegligibletransportof ~3000 kmol
via this route.
Sections
whichresolvethistransport
pathway(e.g., sections
acrossthe LabradorSea) shouldbe incorporated
into any
future inverseanalysisof net inorganiccarbontransport
within the northern North Atlantic.
andRoeromich[1983] and are listedin the captionto
Figure 4.
3. Thereis no netmeridionalflow withinlayersdeeper
than 3000 dbar in the eastern basin of the South Atlantic
8. Summary
The transportof inorganiccarbonacrosssix zonalsections
occupiedbetween11øSand30øSin theSouthAtlanticOcean
hasbeenestimated. During the early 1990s,the transport
wasdirectedsouthward,decreased
slightlytowardthe south,
becauseof the blockingof suchflowsby the Walvisand
Mid-AtlanticRidges.
4. The transportof the Brazil currentat 30øS(vertical
extent
of0-600m)istakentobe10x 109kgs-1southward,
andthe flow of AntarcticBottomWater (04 > 45.92)
through
theVemaChannel
is5 x 109kgs-•northward.
The
HOLFORT ET AL.' SOUTH ATLANTIC CO2 TRANSPORT
497
values of these transportswere determinedfrom direct References
current measurements[Holfort, 1994; Holfort et al.,
Anderson,L. G., D. Dyrssen,andE. P. Jones, An assessment
of
manuscriptin preparation,1998).
the transportof atmosphericCO2 into the Arctic Ocean, J.
5. KeelingandPeng [1995] appliedtheconstraint
thatthe
Geophys.Res., 95, 1703-1711,1990.
net meridionaltransportof phosphate
acrossthe equator Baumgartner,
A., andE. Reichel,Die Weltwasserbilanz,
197pp.,
Oldenburg,Munich, Germany,1975.
shouldequalthephosphate
transportthroughBeringStrait
(about i.7 kmol s-:). Use of a similar constraintin our Bates, N. R., A. F. Michaels, and A. H.-Knap, Seasonaland
interannual
variabilityof oceaniccarbondioxidespeciesat the
inverse calculationsignificantlyrestrictedthe range of
U.S. JGOFSBermudaAtlanticTime-SeriesStudy(BATS) Site,
valid solutionsand better constrainedthe heat, water and
Deep-SeaRes., II, 43, 347-383, 1996.
carbontransports,while only slightlyalteringtheir mean Boebel, O., C. Schmidt, and W. Zenk, Flow and recirculation of
Antarctic Intermediate Water acrossthe Rio Grande Rise, J.
values(Holfort et al., manuscriptin preparation,1998).
Geophys.Res., !02, 20967-20986,1997.
However, it is probablymore appropriateto assumea
balancefor totalphosphorus
in theAtlantic,whichsuggests Brewer, P. G., Direct observationof the oceanicCO2 increase,
Geophys.Res.Lett., 5, 997-1000, 1978.
that the transport of organic phosphorusmust also be Brewer,P. G., andJ. C. Goldman,Alkalinitychanges
generated
by
considered.There are surprisinglyfew measuresof total
phytoplankton
growth,Limnol.Oceanogr.,21(1), 108-117,1976.
phosphorus
in theopenocean,yet dissolved
organicforms Brewer, P. G., C. Goyet, and D. Dyrssen, Carbon dioxide
transportby ocean currentsat 25øN latitude in the Atlantic
of phosphorus
constitute
50-100% of thephosphorus
in the
Ocean, Science, 246, 477-479, 1989.
ocean'ssurfacelayer. Measurementsat the Hawaii Ocean
P. G., D. M. Glover,C. Goyet,andD. K. Shafer,ThepH
Time Series Site [e.g., Karl and Tien, 1997] show dis- Brewer,
of the North AtlanticOcean:Improvements
to the globalmodel
solved organic phosphorusconcentrationsof 0.1 - 0.3
for soundadsorptionin seawater,J. Geophys.
Res., 100, 8761-
8776, 1995.
•mol kg-• withintheupper500 m, dropping
to < 0.05
transportof
•mol kg-• below1000m. If suchlevelsaretypicalof the Broecker,W. S., andT.-H. Peng, Interhemispheric
SouthAtlantic,thereis a potentialnetnorthwardtransport
carbondioxideby oceancirculation,Nature,356, 587-589, 1992.
W. S., T. Takahashi,H. J. Simpson,and T.-H. Peng,
of organicphosphorus
of theorderof 10 x 109 kg S-• X Broecker,
Fate of fossilfuel carbondioxideandthe globalcarbonbudget,
0.25/•molkg4 or 2.5 kmols'• associated
withtransport Science, 206, 409, 1979.
withinthe upperlayersof the ocean.This is presumably Bryan, K., Measurements
of meridionalheat transportby ocean
currents,J. Geophys.Res., 67, 3403-3414,1962.
balancedby a net southward
transportof inorganicphosphorus(phosphate)which is in additionto the phosphate Chen, C.-T. A., and F. J. Millero, Gradual increase of oceanic
CO2, Nature,277, 205-206, 1979.
transportedthroughthe BeringStrait.Withoutdataon the
Coachman,
L. K., andK. Aagard,Transports
throughBeringStrait:
concentrationof organicphosphorus,thereis considerable
Annualandinterannualvariability,J. Geophys.Res., 93, 15535uncertaintyin how to exactlyspecifythe phosphatetrans15539, 1988.
port constraint.Hence duringthe inverseanalysis,three Conway,T. J., P. P. Tans,andL. S. Waterman,Atmospheric
CO2
recordsfrom sitesin theNOAA/CMDL air sampling
network,in
separatevariantsof thephosphate
transportconstraintwere
Trends'93:A Compendium
of Data on GlobalChange,editedby
appliedwhichbounda reasonablesetof possibilities:(1)
T. A. Bodenet al., Rep. ORNL/CDIAC-65, pp. 41-119,Carbon
The phosphatetransportwas specifiedto be identical
DioxideInf. andAnal. Cent., Oak RidgeNat. Lab., Oak Ridge,
acrossall of the SouthAtlanticsections,but its magnitude
Tenn., 1994.
wasnot specified.The phosphate
transportwhichresulted Dickson,R., J. Lazier, J. Meincke,P. Rhines,andJ. Swift, Long-
term coordinatedchangesin the convectiveactivityof the North
Atlantic,Prog. Oceanogr.,38, 241-295, 1997.
phosphateEnting, I. G., C. M. Trudinger,and R. J. Francey, A synthesis
from applyingthisvariantof the constraintwasgenerally
between
4 and8 kmols-• southward.(2) A
transport
of 2 kmols-• southward
across
all sections
was
specified(very similarto the constraintusedby Keeling
andPeng[1995]). (3) A netphosphate
transportof 4 kmol
s'• southwardacrossall sectionswas assumedin order to
balancethe likely net northward transportof organic
phosphorus
as well as the BeringStraitinput.
For the resultspresentedin this paper, mostweight was
assigned
to the4 kmols'• constraint.
inversion
of the concentration
and0•3Cof atmospheric
CO2,
Tellus, Ser. B, 47, 35-52, 1995.
Fu, L.-L., Thegeneralcirculation
andmeridional
heattransport
of
thesubtropical
SouthAtlanticdetermined
by inversemethods,
J.
Phys. Oceanogr.,11, 1171-1193, 1981.
Gammon,R.H., J. Cline, and D. Wisegarver,Chlorofluoromethanes in the northeast Pacific Ocean: Measured vertical
distributions
andapplication
astransient
tracersof upperocean
mixing. J. Geophys.Res., 87, 9441-9454, 1982.
Goyet,C., andD. Davis,Estimation
of totalCO2concentration
throughout
the watercolumn,DeepSeaRes.,Part I, 44, 859877, 1997.
Acknowledgments.
Thisworkwassupported
by theU.S. Depart- Goyet,C., D. Davis,E. T. Peltzer,andP. G. Brewer,Developmentof improvedspacesamplingstrategies
for oceanchemical
mentof Energy(Officeof Biological
andEnvironmental
Research)
undercontractDE-AC02-98CH00016 andby the GermanMinistry
properties:
Totalcarbondioxideanddissolved
nitrate,Geophys.
Res. Lett., 22, 945-948, 1995.
of Science
andTechnology
(BMBF). The support
of MichaelRiches
andT. F. Stocker,An improved
of DOE in allowingus to completethisanalysisis acknowledged. Gruber, N., J. L. Sarmiento,
methodfor detectinganthropogenic
CO2in the oceans,Global
The authorsbenefittedfromdiscussions
withandinputfromCharlie
Biogeochem.Cycles,10(4), 809-837, 1996.
Flagg,AlfredPutzka,PaulRobbins,RalphKeeling,ErnieLewis,
of
JorgeSarmiento,
ScottDoney,Rik Wanninkhof,
andtheDOE CO2 Hall, M., and H. L. Bryden, Direct estimatesandmechanisms
oceanheattransport,
DeepSeaRes.,PartA, 29, 339-359,1982.
SurveyScienceTeam.Assistance
with datacollection
from Rick
Normalmonthlywindstress
Wilke, Craig Neill, andthe scientists
and crew of the Meteoris Hellermann,S., andM. Rosenstein,
gratefully
acknowledged.
Weparticularly
thankTaroTakahashi
and
DavidChipmanfor generously
providing
access
to theirexcellent
SAVE data set. This is a WOCE and JGOFS contribution.
overtheworldoceanwitherrorestimates,
J. Phys.Oceanogr.,
13, 1093-1104, 1983.
Holfort,J., Grossrfiumige
zirkulation
undmeridionale
transporte
im
498
HOLFORT ET AL.: SOUTH ATLANTIC CO2 TRANSPORT
Sfidatlantik,Ber. Inst. Meereskd. ChristianAlbrechtsUniv. Kiel,
260, 1994.
Holfort, J., K. M. Johnson,D. W. R. Wallace, and B. Schneider,
Trans.AGU, 75(3), OceanSci. Meet. Suppl., 161, 1994.
Robbins,P.E., andH. L. Bryden,Direct observations
of advective
Johnson,K. M., K. D. Wills, D. B. Butler, W. K. Johnson,and C.
Roether, W., and A.
nutrientandoxygenfluxesat 24o N in thePacificOcean,Deep
Sea Res., Part I, 41, 143-168, 1994.
Meridional transportof total dissolvedinorganiccarbonin the
Roemmich, D., Estimation of meridional heat flux in the North
SouthAtlanticOcean(abstract),Eos Trans. AGU, 75(3), Ocean
Atlanticby inversemethods, J. Phys. Oceanogr.,10, 1972Sci. Meet. Suppl., 161, 1994.
1983, 1980.
Isemer,H. J., andL. Hasse, TheBunkerClimateAtlasof theNorth
D., Thebalanceof geostrophic
andEkmantransports
in
Atlantic Ocean,Vol. 2, Air-SeaInteractions,252 pp., Springer Roemmich,
thetropicalAtlanticOcean,J. Phys.Oceanogr.,13, 1534-1539,
Verlag, New York, 1985.
1983.
Johnson, K.M., and D. W. R. Wallace, The single-operator
Roemmich, D., and C. Wunsch, Two transatlantic sections:
multiparameter
metabolicanalyzerfor totalcarbondioxidewith
coulometric detection, DOE Res. Sumre., 19, Carbon Dioxide
meridionalcirculationand heat flux in the subtropical
North
AtlanticOcean,Deep SeaRes., Part A, 32, 619-664, 1985.
Inf. Anal. Cent., Oak RidgeNat.Lab., Oak Ridge, Tenn., 1992.
S. Wong, Coulometrictotalcarbondioxideanalysisfor marine
studies: Maximizing the performanceof an automatedgas
extractionsystemand coulometricdetector,Mar. Chem., 44,
167-187, 1993.
Johnson,K. M., D. W. R. Wallace, R. J. Wilke, and C. Goyet,
Carbondioxide,hydrographic,
andchemicaldataobtained
during
the R/V Meteor cruise in the South Atlantic Ocean (WOCE
sectionA9, February-March1991),Publ. 4416, CarbonDioxide
Inf. Anal. Cent., Oak RidgeNat. Lab., Oak Ridge, Tenn., 1995.
Johnson,K. M., B. Schneider,L. Mintrop, andD. W. R. Wallace,
Carbondioxide,hydrographic,
andchemicaldataobtained
during
the R/V Meteor cruise22/5 in the SouthAtlantic Ocean(WOCE
sectionA10, December 1992 - January 1993), Publ., Carbon
Dioxide Inf. Anal. Cent., Oak Ridge Nat. Lab., Oak Ridge,
Tenn., in press,1998.
Josey,S. A., E. C. Kent, D. Oakley,and P. K. Taylor, A new
globalair-seaheatandmomentum
flux climatology,
Int. WOCE
Newslett., 24, 3-5, 1996.
Karl, D. M., and G. Tien, Temporal variability in dissolved
phosphorus
concentrations
inthesubtropical
NorthPacificOcean,
Mar. Chem., 56, 77-96, 1997.
Keeling,C. D., R. B. Bacastow,A. F. Carter, S. C. Piper, T. P.
Whorf, M. Heimann, W. G. Mook, andH. Roeloffzen,A threedimensionalmodel of atmosphericCO: transportbased on
observedwinds, $er., 1, Analysis of observationaldata, in
Aspectsof ClimateVariabilityin the Pacificand the Western
Americas,Geophys.
Monogr.vol. 55, editedby D.H. Peterson,
pp. 165-236,AGU, Washington,
D.C., 1989.
Keeling,R. F., andT.-H. Peng,Transportof heat,CO2and02 by
the Atlantic's thermohaline circulation, Philos. Trans. R. Soc.
London, Ser. B, 348, 133-142, 1995.
Putzka, Transient-tracer information on
ventilationand transportof SouthAtlanticWaters, in The South
Atlantic.'PresentandPast Circulation,editedby G. Wefer et al.,
pp. 45-62, Springer-Verlag,New York, 1996.
Sabine,C. L., D. W. R. Wallace,andF. J. Millero,. Surveyof
CO2in the oceansrevealscluesaboutglobalcarboncycle,Eos
Trans. AGU,
78(5), 49, 54-55, 1997.
Sarmiento,J. L., R. Murnane,and C. le Qu6r6, Air-seaCO2
transferand the carbonbudgetof the North Atlantic, Philos.
Trans. R. Soc. London, Ser. B, 348, 211-219, 1995.
Saunders,P.M. and B. A. King, Oceanicfluxeson the WOCE
All section,J. Phys. Oceanogr.,25(9), 1942-1958,1995.
Schott, F., L. Skamma, and J. Fischer, The warm water inflow into
the westerntropicalAtlanticboundaryregion, spring1994, J.
Geophys.Res., 100, 24745-24760, 1995.
Siedler,G., and W. Zenk, WOCE Sfidatlantik1991, ReiseNr. 15,
30. Dezember 1990 - 23 Mfirz 1991, METEOR-Ber., 92-1, 126
pp., Univ. Hamburg,Hamburg,Germany,1992.
Siedler, G., W. Balzer, T. J. Mfiller, R. Onken, M. Rhein, and W.
Zenk, WOCE SouthAtlantic1992,CruiseNo. 22, 22 September
1992 - 31 January1993, METEOR-Ber., 93-5, 131 pp., Univ.
Hamburg,Hamburg,Germany,1993.
Siedler, G., T. J. Mueller, R. Onken, M. Arhan, H. Mercier, B.
A. King, andP.M. Saunders,The zonalWOCE sectionsin the
South Atlantic, in The SouthAtlantic.' Present and Past Circula-
tion, editedby G. Wefer et al., pp. 83-104, Springer-Verlag,
New York, 1996.
Stoll, M. H. C., H. M. van Aken, H. J. W. deBaar, and C. J.
deBoer, Meridionalcarbondioxidetransportin the northern
North Atlantic, Mar. Chem., 55, 205-216, 1996.
Takahashi,T., R. A. Feely, R. Weiss, R. Wanninkhof,D. W.
Chipman,S.C. Sutherland,andT. T. Takahashi,Globalair-sea
Lundberg,L., andP.M. Haugan,A NordicSeas- ArcticOcean
fluxof CO2:An estimate
basedonmeasurements
of sea-air
pCO2
difference,in NAS ColloquiumVolumeon CarbonDioxideand
carbonbudgetfrom volumeflows and inorganiccarbondata,
ClimateChange,editedby C.D. Keeling,pp. 8292-8299,Natl.
GlobalBiogeochem.
Cycles,10(3), 493-510, 1996.
Acad. Sci., Washington,D.C., 1997.
MacDonald,A.M., Propertyfluxesat 30øSandtheirimplications
Tans, P. P., I. Y. Fung, and T. Takahashi,Observationalconfor the Pacific-Indianthroughflowandtheglobalheatbudget,J.
straintson the globalatmospheric
CO2 budget,Science,247,
Geophys.
Res.,98, 6851-6868,1993.
MacDonald, A.M., and C. Wunsch, Oceanicestimatesof global
oceanheattransport,in US WOCEImplementation
Report,Rep.
1431-1438, 1990.
Wallace, D. W. R., Monitoringglobaloceancarboninventories,
report,54 pp., OceanObs.Syst.Dev. Panel,Tex. A&M Univ.,
8, pp. 28-30, TexasA&M Univ., CollegeStation,Tex., July
1996.
CollegeStation,Tex., 1995.
Martel, F., and C. Wunsch, The North Atlantic Circulationin the
Wallace, D. W. R., P. Beining, andA. Putzka, Carbontetrachloride andchlorofluorocarbons
in the SouthAtlanticOcean,19øS,
early 1980's- An estimatefrom inversionof a finite-difference
J. Geophys.Res., 99, 7803-7819, 1994.
model,J. Phys.Oceanogr.,23, 898-924, 1993.
Neftel, A. E., E. Moor, H. Oeschger,andB. Stauffer,Evidence Wallace,D. W. R., K. M. Johnson,J. Holfort, andB. Schneider,
Detectionof the changingCO2 inventoryin the ocean,in 1996
frompolaricecoresfor theincrease
of atmospheric
CO2in the
U.S. WOCEReport,U.S. WOCEImplement.
Rep.8, pp. 31-33,
pasttwo centuries,
Nature,315, 45-47, 1985.
U.S. WorldOceanCirc. Exp. Office,Dept.of Oceanogr.,
Tex.
Perry,G. D., P. B. Duffy, andN. L. Miller, An extended
dataset
A&M Univ., CollegeStation,Tex., 1996.
of river discharges
for validationof generalcirculationmodels,
Watson,A. J., P. D. Nightingale,andD. J. Cooper,Modelling
J. Geophys.
Res., 101, 21339-21349,1996.
atmosphere-ocean
CO2transfer,Philos. Trans.R. Soc.London,
Reid, J. L., On thetotalgeostrophic
circulationof theSouthAtlantic
Ser. B, 348, 125-132, 1995.
Ocean:Flow patterns,tracersandtransports,
Prog. Oceanogr.,
Wunsch,C., The North Atlanticgeneralcirculationwestof 50øW
23, 149-244, 1989.
Rintoul, S. R., and C. Wunsch,Mass, heat, oxygenand nutrient
determinedby inversemethods,Rev. Geophys.,16, 583-620,
1978.
fluxesandbudgetsin the North AtlanticOcean,DeepSeaRes.,
Wunsch, C., D. Hu, and B. Grant, Mass, heat, salt and nutrient
38, suppl.1, S355-S377,1991.
fluxesin theSouthPacificOcean,.J. Phys.Oceanogr.,13, 725Robbins,P. E., Direct observations
of the meridionaltransportof
753. 1983.
totalinorganiccarbonin the southAtlanticOcean(abstract),
Eos
HOLFORTET AL.' SOUTHATLANTICCO2TRANSPORT
499
Zenk, W. and T. J. Mfiller, WOCE studiesin the SouthAtlantic,
Cruise No. 28, 29 March - 14 June 1994, MEIEOR-Ber., 95-1
193 pp., Univ. Hamburg,Hamburg,Germany, 1995.
J. Holfort, BrookhavenNationalLaboratory,PO Box 5000, Dept. of
Applied Science, Upton,NY 11973-5000
D.W.R. Wallace,Brookhaven
NationalLaboratory,
PO Box 5000Dept.
of AppliedScience,Upton,NY 11973-5000 (e-mail wallace•bnl.gov)
(ReceivedOctober14, 1997, revised,April 22, 1997;
acceptedMay 1, 1998.)